It's actually not as bad as you think. The lack of power in the Wii is a problem, but that itself not what's causing lack of third party support.
The problem with the Wii is that it uses the same tech as the GameCube. The GameCube itself was unusual even at the time of its release, because it didn't follow the same shader structure as the PC and Xbox. Even during the GameCube time some third party developers simply couldn't make sense of it and simply left shaders out. Incredibly, and despite increasingly shader heavy games, Nintendo didn't do anything about it used the exact same tech in the Wii.
Nintendo could have created a console with the same computational power as the Wii, but instead supporting technology other people were using at the time. It would have allowed the Wii to run id Tech 4 (Doom III engine), Source (HL2 engine), the CoD engine to some extent and maybe even a watered down UE3.0. Because it didn't, it meant that developers had to write entirely new technology for the Wii. That's an investment lots of them didn't want to make.
The point is, that's not going to happen with Wii U. It has tech that at least conforms to DirectX 10, and probably DirectX 11. It will be at least possible to port games, opposed to having to write entirely new games from scratch. Moreover, there is no way that Sony's and Microsoft's offerings are going to make as big a leap as they did last time (at least not when launching in 2012), so there's absolutely no way the Wii U will be in the same position.