Simple, really. Most games don't have integration for it and so you have to use third party software to my understanding to make it work with your game. I'm not sure if you have to go out of your way for drivers or not. It's not like it's impossible or anything. But with 360 controllers, on Windows PCs at least, it's a simple plug and play for most games.
But I think Playstation controller players are a minority. I'd have to think so, since no game seems designed with them in mind outside of emulators.
Gianni
Some folks really like the run Playstation controllers but they have to work harder for it. Large majority rolls with a 360 pad it seems. I prefer mouse and keyboard but on games where it's smoother or more logical to use a controller, I also go with the 360 controller.
BoMToons
What do the Playstation controllers have to do extra to work with most games?