[grumpyfart]

Many reasons:

Look at the sales of PC games and console games. It's clear that PC games are going down. Many companies don't even release PC versions any more.

PC gaming is more expensive, you got keep your hardware up to date and that's truly expensive. A good graphic card's price is more expensive than an XBOX 360.

Piracy is a big issue in gaming and piracy rate in console is much lower (none in PS3, although potentially next year we'll see PS3 as well).

Developing games for a fixed hardware is much easier unlike developing games for a platform such as PC or God forbid, Linux.

PC is going to a be only niche players complex MMORPGs, indie game developers, some RTSs.