Because I don't have the moneys too build a computer/buy a new computer that's able to run today's games and most of my real life friends and people I know play on the Xbox 360.
My parents won't allow me to build a new computer. I did tell them I could use our desktop's tower, and they seemed to warm up to the idea a bit more. I prefer PC gaming more because it lets me mess around with the games as opposed to being stuck just guessing why something won't work. I can play games of all languages, as well. If I want to play a PC game from Japan, I could with no issues. I also like how there's more games that cater to me.
I'm the kind of person who likes more complex games and likes to have more freedom in how they choose to play their game. Consoles are getting better now with that, but I still would rather have less restriction. I don't see it as the only way to game, just the kind of gaming that is more for me.