The excuse last year was that the developers didn't anticipate the success of the Wii, so they didn't devote resources.
Now it's 6 months later and the serious properties in development for Wii, like Ghostbusters, are getting short shrift -- no online multiplayer, inferior (PS2) engine, etc. Most 3rd party Wii titles are bad ports or "casual" games (none of which are selling well).
Hasn't Wii peaked? Wii was an excellent sprinter, but it is failing in the marathon.