1) Volume of ammo sold. The bird shot is sold in MUCH higher volume.
2) Ease of manufacture. Running shot through the tower is easier than swaging slugs where amount by weight is concerned.
3) Ease of manufacture again. It is easier to load a one piece plastic wad, drop a load of shot, and fold crimp on automated equipment than it is to drop a series of cardboard and felt wads, drop in a single slug and roll crimp.