Publishers go where their games can make money. EA tried to do it with Dead Space: Extraction which is an incredible innovative light gun game but it sold poorly. Zack and Wiki from Capcom used the Wii-Mote in incredible ways but it sold poorly. Madworld is the most brutal mature title on the Wii, it sold poorly. There games just don't sell well.
Nintendo develops hardware significantly less powerful these days compared to the competitors. There would be a lot of sacrifice to be made to bring a lot of these third party game on the Wii and now Wii U. Another factor I would consider is that the core audience on Nintendo's systems just want Nintendo franchises. I think there is a market for third party games but its very risky, especially now when their hardware is selling terribly.
