The reason is that 3rd parties idiotically listen to the hardcore gamers and use their impressions of Wii to gauge their support of the console.. And since Wii is not exactly the hardcore's favorite, the developer's support is influenced by their opinions, rather than the opinions of the mass market and sales trends. Developers, like many hardcore gamers, are so out of touch with reality it's scary. This explains why one of the highest selling consoles of all time has recieved 3rd party support equal to that of one of the worst selling consoles of all time.
Hopefully they will wake up next gen, and listen to Nintendo and the mass market, rather than foolishly being influenced by a small minority of very vocal hardcore gamers, who are just as out of touch with reality as they are. As I've said before, it you refuse support of the market leader, you do so at your own risk.
