This question should be proposed to the developers and i like to see what they say. I personally think the developers think nintendo is more of a risk, because of the motion controls, and they don't know who the nintendo audience is. Many people think like the developers that nintendo audience is kiddy or what have you. But they should beaware that a lot of core gamers have multiple systems and they would gladly buy a wii game if it is good and not some port or rail shooter. They could get the better version on the other platform. It seems to me that the developers don't want to spend to much time on motion controls, because with ps3 and xbox360 controls are the same and been the same for a while. The wii u have to do so much more with the controls and to make the controls right might take a long time to master, that could be one reason why.
If developres put core games out on wii system like bioshock, assassins creed, left for dead, re5, and codmw2 and make the wii controls work well i think the games will sell well. Their is only one way to find out and that if developers have the cojones to do it.