Here's why Smash Bros works and these other games fail:
1: It was made by a team who both knew how to make a platformer and a fighting game.
Sakurai notably has done several games for the Kirby series and is a veteran fightan player, given a lot of moves in Smash reference Street Fighter and how Squad Strike is just a KOF mode. All Star Brawl 2 has the most fightan DNA in it with roman cancels and a clearer differentiation between light and heavy attacks. There are characters that clearly show a love for the fighting game genre as well, Plankton, while taking aspects from Bowser and King K. Rool, has many elements from Potemkin and the Angry Beavers work like a MvC duo. Multiversus devs have mainly worked on League of Legends, which shows in how many cool downs the characters have, when in a typical fighting game powerful moves would rarely be balanced with cool downs and rather with high start and end lag. The elements of MOBAs work better with a slower paced game but fighters are inherently fast and phrenetic, causing the two elements to clash hard.
2: Every character plays the same.
All the characters in Multiversus have the same combo game: using jabs to rack up damage and then spam one good move to end a string. All Star Brawl had the same dilemma where every character had the same combo of up-tilt and dair. While your average ASB character is a shittier Sheik, your average MVS character is a shittier Jigglypuff. Giving every fighter the same fall speed might sound good on paper, but it ends up causing people to get more predictable with combos, thus repeating the cycle.
3: They always try to focus on the competitive side and neglect the average player.
This kind of reaction, according to the original Reddit leaker, was done as a response to Nintendo not letting the competitive scene flourish. Even though that was not the clear intention, even from the beginning. Am I saying we should let go of the aspect entirely? No, especially when every update raises one character into being too broken and unwieldy for any other player to counter. It's just that more focus is taken towards the competitive side that a lot of more entertaining aspects are shafted. Items have been left in the dust (minus Shaggy's sandwich, but that's like saying Peach's turnips are items), nearly every stage is a variant of Battlefield/Final Destination, all the casual modes are stuck in the godawful Rifts, and events like Wacky Weekend are only there for a limited time before being locked for a few days. Smash Bros has a lot more variety with stages and has plenty of modes. For as much as developers of Smash clones like to take inspiration from Melee, they somehow avoid things like event matches, Break the Targets, and All-Star mode even though those were staples of every game until Ultimate, which did away for those modes in favor of pleasing rosterfags. And speaking of...
4: The rosters aren't interesting or make a lot of sense.
It's been very clear that for the last few Smash games people care about the roster more than anything else. That's the key draw to these games, and anyone who says it's because of the gameplay is delusional, as trash like Fortnite is popular despite every crossover being a skin and the game being a battle royale. People will eat any slop as long as it shows whatever recognizable thing they have fondness for, Smash and its imitations are no different. PlayStation All Stars suffered from a lack of enough icons and a bunch of mandates. While having icons of the console like Kratos, Ratchet and Clank, Sackboy, Nathan Drake, Spike and Heihachi are present, there are many odd choices. Why add Big Daddy, a character more associated with Xbox? Why have both Good and Evil Cole fill up 2 slots? Why get Dante but make him the shitty reboot version that no one liked? Why get Raiden instead of Solid Snake? Why miss out on classic icons like Crash, Spyro and Cloud Strife? A lot of those choices can be answered with the fact that Sony and various other companies wanted this game to advertise the current thing rather than have it be a celebration of PlayStation history like Smash Bros is with Nintendo. And given that many more picks like Tomba, Abe and Dart Feld were scrapped due to lack of interest, the game was this close to living up to whatever potential it had.
Nickelodeon All Star Brawl 1 and 2 also suffer from this, though in different ways. As a few people have mentioned on /v/ and /vg/, the first game was mainly focused on the 90's Nick, with many key figures from later on notably missing. It took until DLC and clear demand for us to get a character from Jimmy Neutron and My Life as a Teenage Robot, after all. Plus the game had no voice acting at launch, making every character feel lifeless. The sequel tried to fix a lot of the issues, but more often than not it traded 90's characters with 2000's characters, and usually in a way that felt more like an equivalent exchange rather than an addition given they had to cut half the roster. Even not bringing in decade representation it still felt that way. Trading Sandy for Squidward, Toph for Azula, CatDog for the Angry Beavers, and replacing Leonardo and Michelangelo with Donatello and Raphael. Plus the only true 2010's rep, The Loud House, lost its main character but kept Lucy simply because of her gameplay rather than her significance. This kind of cut and paste method is the main reason this game did not sell, especially with casuals. The DLC was also doubling down on already established and well-represented franchises instead of new blood. While Mr. Krabs and Zuko felt like highly requested characters with unique enough kits, Iroh was unnecessary and gave Avatar 3 Fire Nation reps in the place of our only Earthbender, and Rocksteady, while having a few good moves, is not important enough to replace the other 2 Turtles and Shredder.
Multiversus had a good start but once the game released it's clear it was just a vehicle for Warner Bros to shill what new products they could find. LeBron James is a good example of this, no one asked for him and we all knew he was a shoe-in solely due to that awful Space Jam sequel. Rick and Morty were released around the time Season 6 of the show was coming out, with one of Rick's moves being from that season. The most obvious one was Black Adam, who was only here because Warner Bros wanted his movie to make enough money to get them out of their self-imposed debt. Him being released before many more recognizable and interesting DC characters was enough of a sign, it didn't help that his "reveal trailer" ended with an ad for the movie.
When the game shut down we all thought that they would learn from this, and from the looks of Season 1 they were. The exception being Banana Guard, a character NO ONE wanted and had no unique moveset potential, instead being a generic spear wielder with none of the guards' personalities and quirks from the show. Season 1 did have good picks like the Joker, Agent Smith, and especially netting Jason Voorhees after the legal settlements were over with, but it seemed that the cracks were beginning to show.
While I would say Jack and Beetlejuice were needed for the roster, I feel that the Ghost with the Most only got more priority because of the new movie coming out, easily shown by that poor man's Sakurai Presents (we'll get to the marketing later) ending with the first trailer for the movie completely unedited. While Beetlejuice is fun and his kit is inspired, it's clear that they didn't add him out of variety and more out of necessity. Season 3 gave us the Powerpuff Girls, who like the Joker should've come a lot sooner than they did, but shoehorning in Nubia simply because DC wants to push her is a bad move. She's not an obscure fan favorite character like Krystal or Geno, she's not an important character in comic history like Hero is to RPGs and Terry is to SNK. She's there to fill a diversity quota and to shill the plenty of new comics that DC has planned that feature her. In a game with wild and varied personalities she stands out the least, and that's surprising because even the mute killing machine that is Jason Voorhees has more charisma. Season 4 looks like it could either be an improvement or letdown, but looking at it objectively, adding another AT and DC rep when the franchises are already represented better than the fucking Looney Tunes, the MASCOTS of the damn company, feels wrong. It's almost like a Nintendo game with Fire Emblem getting more attention than Zelda or Mari-OH WAIT.
5: The marketing/advertising is piss poor.
Think about how Smash Bros was initially shown. A commercial of characters like Mario and Pikachu beating the shit out of each other in amusingly violent ways. While Nickelodeon and Multiversus have tried to do that, it lacks the heart and soul that the original commercial had. The only thing that came close were the 2 cinematic trailers Multiversus had that tried to make a narrative out of it, but those only happened once in a blue moon. The character showcases and trailers all blended together, having the same formula, while when Nintendo reveals a character, they tend to go all out and show the character's personality and how it clashes with others in the game. Sephiroth doesn't need to have the same introduction as Isabelle. Simon Belmont doesn't need to have the same introduction as Banjo-Kazooie. They do what fits best for the character and the tone fits the franchises they hail from. Every NASB 1 trailer is the same "GET READY, GET READY" bullshit with the same gameplay with no thematic ties, every NASB 2 and MVS trailer is just the character saying quotes from their source in such a stilted and unnatural way. The gameplay footage isn't even interesting, it's the same "hype" combos you'd see in a shitty montage YouTubers would churn out. Even the gameplay segments in the trailers were meticulous and would reference the characters' personalities and iconic moments relating to them. Things like King K. Rool trying and failing to steal one of the banana bunches on Tortimer Island or Ken's reveal ending with him performing EVO Moment 37 on Little Mac adds a lot to the presentation. Here, we get none of that.
Everything blends together and just using in-game footage can only get things so far. What's baffling is that they do use actual cinematics but they're all in Season 1. We don't get any explanation outside of the awful Rifts writing.
Even then, other forms of marketing are bad as well. While Happy Meal toys are a good way of keeping something in the public consciousness, who asked for a hockey game? There's barely any overlap between audiences, and even if this was supposed to be some soft-reveal for Jason (it likely was given development for characters takes a while) it certainly didn't fit. Even the Nickelodeon Super Bowl had more to do with it if only because Sweet Victory was associated with the event in recent years.
The "Into the Verse" streams are bad as well. Ajax and Nakat have no form of charisma compared to Sakurai and they don't feel like they really get the characters. The presentations of the characters once again get into the mindset of being too competitive, and sometimes they don't explain crucial information like how they conveniently managed to avoid talking about Beetlejuice being paid for a few days.
6: They put payers first over players first.
The most egregious aspect of Multiversus, and perhaps its most infamous drawback. Ever since the relaunch, the game has been doing its best to nickel and dime players who aren't grinding this game like a second job. There are 3 (once 4) currencies, and only one of them actually matters now, of course the one that is most associated with real money. A lot of events feature skins which can only be unlocked with an exclusive currency - that is in limited supply unless you manage to buy it. No money? No time? Get fucked! It got even worse in recent events where the event currency was limited and certain "elusive" variants were only available by paying with the max amount of exclusive shekels. That's not even counting how little it used to give in return, something the team only backed out on because everyone said it was a retarded decision. But still, the greediness of the game still stands tall, and they'll find new ways to scam idiot whales and dicksuckers out of their money. Sad, but this is the state of gaming now. You WILL enjoy paying for useless shit, and you WILL be grateful for every backstabbing move that Warner Bros will do against you.
Overall, this game needs to be a decent contender to stand up to Nintendo, but it seems like every aspect is working against that.