I think third-parties are stuck between a rock and a hard place this generation. If they make a top-tier, AAA, "expensive" Wii game, it would be unique on the platform but might not sell regardless of their effort, since the buying habits of the Wii audience is a little hard to predict. If they make a top-tier, AAA, "expensive" 360/PS3 game, it will not be unique on those platforms, but it will *probably* sell because the buying habits of those audiences are more predictable.
I think third-parties are reticent to dive into Wii development whole-hog because very few companies besides Nintendo have really put up huge numbers. You have your Just Dance titles that do well, but those "casual" genres get saturated too. And people looking for the more "hardcore" titles probably already have a 360 or PS3 anyways, and abandoned the Wii long ago.
In a landscape of gambles, the Wii ironically seems to be MORE of a gamble, since the platform has been pigeonholed as a family-oriented console by gamers and press alike. I think perception has become reality on Wii, and its hard to reverse that sentiment once it's become entrenched.
<Side rant incoming...>
Nintendo also hasn't done anything to really cultivate audiences in the more "hardcore" genres either. Why not have the Wii equivalent of Goldeneye 007? Why is the new installment of Golden Sun going to DS instead of Wii? It doesn't seem like Nintendo themselves even have any faith in those genres on their own console, or even care about growing those genres on Wii. It's not the job of third-parties to test out the waters for a genre on a console; that should be done by the console-maker, since they have the money to incur the most risk. Nintendo would help out a lot by at least having some flagship franchises that don't involve Link, Mario, Samus, or Miis.
Why Nintendo doesn't have a Dragon Quest-type RPG franchise, I'll never know. They could do it as well or better than anybody else. Somebody will say, "Oh, but JRPGs are a dying genre, Nintendo doesn't want to make a money sink, red ocean blah blah blah" but come on. The 3D platformer could be considered a dying genre, yet Nintendo still creates amazing Mario games that set the industry on fire. The reason why the 360 is huge for first-person shooters is because Microsoft made it a priority to cultivate the Halo franchise. They created an audience for first-person shooters on their console out of thin air, and third-parties have been feeding off of it ever since. Until Nintendo steps outside the boundaries of their general-interest fare, then the third-party output for the console will cater mostly to the audience that those general-interest games create. Granted, that's a big audience, but not the total audience.
<Side rant finished>