The wii is aimed at a different audience. Had it not been for casual gamers, and parents wanting to get their kids "a non violent system" the Wii would not have sold as much. With that in mind, most of the market, and limited hardware of the Wii prevent it from seeing a lot of titles.. Others get a completely different game from the HD consoles, just, use the same name... Like... Sonic Unleashed.
Had the Wii had the audience, and hardware to support the games, or, at very least, the audience, it would have most of the games. That's why a lot of games don't sell on Wii.. People who wanted the game already have a 360, or a PS3.