Because they're either niche, poorly thought out, or totally suck?
There has still yet to be a single AA 'hardcore' 3rd party game released on the Wii. The closest thing is Resident Evil 4, a port of a game that could already be played on the Wii. GH3 is also good, the first multi-platform where genuine effort was put in - and guess what - it sold very well despite having no history on the platform like other versions did!
I can't believe anyone seriously expects broken PS2 and PSP ports, poorly thought out marketing disasters like Dewy, hard-sell poorly positioned niche stuff like Z&W, rushed efforts like Sonic and budget crap like Carnival games to sell like mega-budget, polished, years in the making AAA super-games like COD4, Bioshock and Skate.
When the Wii receives a single 3rd party game as good (or at least as well done) as COD4, Bioshock, Assassins Creed, Lost Planet etc, and it bombs, then you can claim 3rd parties don't sell on it.
Until then there is no evidence.