When you try to make a 2D platformer fighter, you have to REALLY know what you're doing. Time and time again these things fail, even if they have new mechanics, because they forget about one thing or another, or their movement isn't as smooth, or the balance is totally messed up, or like what may be the case here, their payment model is just bad and unsustainable.
I think MultiVersus has a kind of cool idea, but it's horrible for 1v1 (being that it was built for 2v2) and I find the UI pretty messy, which is what is typical for one of these live service games. The movement and flow of the game is also nothing like Smash so it's hard to get into it for me personally. It doesn't feel anywhere near as polished, and the Nick All Stars game had that issue and died pretty quickly too. With Smash being such a monolith, you have to give people a reason to play your game over Smash, and if the reason isn't there or isn't good enough, good luck.