This rule is required because "as in chess", a pawn may promote to bishop.

If you only remove the bishops from the starting position, then the rules of chess still allow bishops to come in later in the game (through a promotion).

I am assuming they did not have enough memory to hold the rules of bishops, and therefore decided to completely remove them from the set of possible pieces. They allow rooks and knights promotion because these pieces are in the set of initial pieces: their rules are already implemented.