According to Sakurai's Famitsu column, he chose to build the game off of the previous entry's mechanics instead of building the game from scratch as choosing the later would have caused a large portion of the game's roster to be cut. This would be due to the fact that more time would have had to be put into rebuilding each individual character.
He also remarked this decision allowed for more focus to be put on other aspects of the game.
Contributed by Boyobmas