It might just be the Wii's control scheme and that 3rd parties just don't know how to deal with it. Which is stupid, because the Wii and Nunchuck CAN work as a traditional control scheme rather easily, and if that is still an issue then there is always the classic controller.
So its stupid, but I think this might be part of the reason 3rd parties fear the Wii. No one can claim the DS or PSP are graphically superior systems to the Wii, and yet these systems are getting games like Assassin's Creed and GTA and so forth. Hell, even the PS2 is probably getting more 3rd party support at this time than the Wii is.