Except for the width that they cover and how much of a pile they create in each variant...
True but the five tetrominoes are _the_ free polyminos with four unit squares... they are a mathematical fact, other four unit square shapes are all derivatives and since in this game you can rotate them a number of those derivatives are even included. But I suppose you could go for the five unit square polyminos, but there are a lot more of those and therefore are harder for the player to remember how they would like to deal with each and the three unit square polyminos are just too few and compact.