Duhhh, people are just buying them to replace their broken ones ;)
Seriously though, this is just a silly question. Cheap system, cheap games, tons of games, small system. That's all.
My End of 2008 Hardware Predictions (console only):
Wii : 50 million
360: 28 million
PS3: 24 million
These predictions were made on January 3rd and won't be revised