As a new player, the reason is obviously combos.
If I've been waiting for a few minutes to be finally be able to make a move, and then the other player lands their free completely unavoidable hit and I'm suddenly 40 frames behind again, why shouldn't I quit?
At that point I'm not playing the game anymore, so there's no reason to bother.
Seriously though, recovery times in this game are just waaay too long considering the fact that the advantaged player has plenty of time to figure out how to sustain their combo and the disadvantaged player has zero counterplay besides being able to slightly nudge the direction in which they're juggled.
Also I'll quit immediately if the other player picks wizard, there's no reason I should have to put up with that nonsense.