Because the crossbar is way too limited a control scheme for hardcore playing. Sure the proud desparate few skilled PS3 players could get it down, but anyone could play better on PC than they could on PS3 hands down. Targeting on PS3, even with filters set up just right, is still a fetid pile of garbage.

Because the frame rate on PS3 is horrid.

Because the majority of PS3 players don't even use a keyboard with their system at all.