third parties already abandoned nintendo after gamecube sales. the unexpected success of the wii forced them to make games for the big n cuz nobody wants to miss out on easy cash. then they used all the money to develop for the hardcore fans in the other systems which unfortunately forced a lot of them to go bankrupt due to high costs and not enuff sales, and others to blame Nintendo for existing. wii u is not replicating the wii's success so third parties are just going back to their original plans.
I added one more thing.