Because modern games sell through hype. Nobody would pay money for 99% of PS2 generation games now, because the graphics are terrible now and the gameplay was never that great. The games people would pay money for are getting sequels and updated re-releases for current-gen systems with current graphics.
Also, PC gamers are notorious pirates. Devs don't want to spend money to make a product which will be stolen.
Wii has more 20 million sellers than PS3 has 5 million sellers.
Acolyte of Disruption