The only thing that I can think of is that SuperBot wanted to focus more on getting the game right, than trying to add 40+ characters to the first All-Stars game and trying to figure out how to balance them all.
Gameplay needs to come before quantity of characters. This is the studio's first game and it would be terrible if it was a horrendous game. From what I've seen recently, they are fixing the art style so there isn't so much clash and I read somewhere that the camera/stages are/were being re-tooled.







