Coming from the Kickstarter page:
A lot of you are asking why we aren't adding Wii U as a stretch goal, and to be honest, we really want to! But unfortunately, despite the Nintendo eShop's ability to self-publish, our game engine does not support Wii U with the license that we have!
That is why we might need help getting the game on other platforms than Windows and Mac! As you can hear, it is quite a rut we're in, but we'll definitely let you know when we are ready to support Wii U! We really want to make it possible during the Kickstarter period! Rest assured that we'll do everything to get on the Wii U!