The wii audience and nintendo audience in general is allot different in gaming tastes then what most 3party´s (especially western ones) make these days.
As some of you have said this started in the n64 era, the era were Nintendo started to detach itself from the rest of the competition, Nintendo gamers (the majority not all ) don't like fighting games they like smash brothers, they don't like racing games they like Mario cart they don't like shooters they like metroid, they don't like adventure games they like Zelda, not to mention platformers were Mario reigns supreme
My point is, Nintendo gave its spin to most popular genres, that's so different to traditional 3party stuff that if they are not similar to those nintendo games most people will just ignore them as being lesser games
On the wii Nintendo took this approach a step further adding to its traditional core franchises new ones that are even more detached from the traditional 3party way of making games, not only that but they also added a new control method that like it or not is also a big factor, it means that they just cant port a game from PS360 they have to make a new version that will probably sell very little compared to them
some are starting to adapt like EA and Sega they have realized that to grab a piece of the pie on the wii they have to change the way they think about games to some degree, one is already doing really well ( Ubisoft ) they adapted right after the wii launch because of the experiences they learned on the DS and moved some of there franchises right away to the wii with great sales for products that many here hate but are what a big part of the wii audience wants
So in my opinion don't blame the 3party´s blame nintendo for being to detached in this generation from their way of doing business, they are adapting to the wii just don't expect to see all major franchises flock to the wii because that wont happen and its not even the point of the wii as a console
There is another solution buy another gaming system ;)