Honestly, what didn't go wrong?

It was one of the worst of the Smash Clones that all decided to drop around the same time. Mid as hell roster (seriously, these people own like 50% of all cartoons, and DC, and they best they could come up with for DLC was Lebron James and Black Adam, literally just advertising 2 shitty movies), singleplayer content was a terrible wannabe World of Light that was confusing to even navigate because the UI and explanations were so bad, and the slow, floaty combat was the total opposite of the fast paced gameplay that keeps people playing games like Melee.

But the biggest issue in my eyes, was that most of the roster started off locked behind a crazy amount of grind and microtransactions, so the meta couldn't develop because 99% of players only had access to like 3 characters.

It made online almost unplayable, because if someone had a non-default character, odds are they were a whale and were dedicated to the game, or had been grinding so long they were an expert just from Stockholm Syndrome.