Because the CPU is garbage. Underpowered, less efficient in nearly every aspect compared to the PS3 and 360
And part of the problem is that it's actually only a *bit* less powerful. They could have recoded the game to take some of the CPU work over to the GPU (which probably isn't being stressed), but given that they're only running a handful of frames slower... and given that it's a port, etc etc...
Just leave it as is. It's a lot of work to get a game to run marginally faster. In games like this where the game only needs to run marginally slower to go from passable to not good enough, you're left with an ugly final product.
If the CPU was weaker further still, they wouldn't be able to run that same code on the CPU and they'd have no choice but to move work over to the GPU... as it is, they just figure most people won't notice.
Maybe they're right, but it's still disappointing. All the Wii U needed was a *slightly* more powerful CPU that could at least run code designed for the 360 as well as the 360 could. Then Wii U ports would be equal or better in almost every case.
We've seen what a really talented team putting in a lot of effort can do (Bayonetta and Most Wanted as mentioned), but Wii U versions aren't going to get the budget to support the amount of man hours the current hardware requires.
If the CPU was equivalent to the 360... they wouldn't have to rework the whole engine just to get it running as well as it does on much older hardware. Frostbite would have worked on it just fine and the system would have seen more EA games. Etc etc.