Publishers have fairly complicated models, that are a lot like the models created by insurance companies, in order to handle the risk associated with producing a game for a platform. These models take into account attach rate, userbase size, sales rate, the performance of similar titles (in quality, content and genre), and development costs.
Basically a system like the Wii that has a decent userbase that is growing rapidly, with a healthy attach rate and low development costs really has very little to worry about when it comes to third party support; until publishers start producing high quality games in popular genres that perform poorly on the Wii, I don't see any reason why someone would look at the numbers and refuse to produce a Wii game (in favour of an XBox 360 or PS3 game).
