Two words: Couch Multiplayer.
I know it doesn't matter much to you, but when you and your friends get smashed, are you going to want to get frustrated by an FPS, RTS,or even a sports game? Well, I'll tell you the people who have to endure listening to you certainly won't. Meanwhile; Wii. Simple, easy, 'fun' games that you can play with any mind-set. Think about other people.
Oh, and the fact that the Wii outsells both Xbox and PS3 combined doesn't hurt.