Well after the N64 the 3rd parties, except for Capcom which was using Nintendo as a life line since their business was financially in trouble at the time, didn't want to bother for the most part with the Gamecube. But that started to turn around right before Nintendo made a stupid manufacturing mistake and made way too many, and had to stop making them. (In response to oniyide.)
Adding a little bit to what I was saying before. The game was developed right beside the PS3/Xbox 360 versions which means it was done on the older SDK units, which rumors say was given a significant power boost at the beginning of the year, which may also have lead to them not wanting to changing things late in the development of the title. As said, THQ is cash straped right now and them just making a game for the new system should be a nod that they believe in what Nintendo is bringing to the table.
http://wii.ign.com/articles/122/1221515p1.html







