I would like to ensure you all that we are still working on the Wii U version. We did the first optimization pass in January and were very hopeful that we would be able to release the game soon after that. However during the Quality Assurance process afterwards, we concluded that it needed more work, and so here we are.
The Wii U version has gone through a full second optimization pass and is now in QA again. We are still working hard on getting it out ASAP, but having missed the estimated date in the last KS update, I'll just stick to the "as soon as possible".
Many of our Wii U backers have been asking us if we can give them the option to instead choose a Switch version. If and when we release a Switch version we have decided that all Wii U backers will get both. So feel free to get your Wii U version as soon as it arrives!
(We will announce news about a possible Switch version at a later time.)