The most likely reason is this:
-Programming of the product is close to completion and only tweaks, optimizations, and other polishing touches are left to make the game better... and it would take a lot of reworking to get it to port to the Wii-U. That, and the Wii-U's very small current user base would not warrant the resources being used to port it.
It's the only understandable reason I can think of.
I don't think the company's like "Oh the Wii-U sucks, let's not make games for it". That would not be a good business thing to do.