Why is anyone surprised by this? If I were a game developer i'd certainly rather develop on the PS3 or X360. Developers want to develop games they want to play and given the demographic game developers tend to fall in you begin to see why they want to develop HD graphic high intensive games. The few developers that would like to develop the type of games that excel on the Wii would have to _compete_ with Nintendo games and that's never going to work.
Can you imagine some studio coming out with Super Alex Bros.? Not a bloody chance.