If you make good games, they will sell. The 360 just has more good games. Partly because it's been out a year longer, partly because Microsoft bought some of them, etc...
Metacritic, based on a score 75 or abovE:
360: 212
Wii: 66
PS3: 125
The bigger question... why isn't the 360's attach rate 3 times as high as the Wii's, given the difference in number of games worth attaching...