it's probably a conspiracy.
think it about it... nintendo has profited ridiculously from the Wii. if nintendo is the only company releasing good games for its system, well then it's going to profit even more ridiculously.
if third parties dont make decent titles for the Wii, then nintendo stands to make even more money. I'm sure 75% + people who own a Wii will by Super Mario Galaxy, Super Smash Bros. Brawl, and Mariokart.