We've now had an update on why that was the case, as well as a revised release window of Summer. It seems that the game has hit some snags when it comes to passing Nintendo's Lot Check, with one of the extra features making approval difficult. This undisclosed feature may need to be removed in order to pass the check, but developer Matthew Taranto has stated that he will find a way of getting it back in, post-launch.
Here's the update from the Kickstarter page:
Of course, the question in mind for many of you is regarding the Wii U version's release date. To be straight up, we're having quite a bit of difficulty passing through lot check, and it turns out that one of our extra features has been making this task particularly difficult. Unfortunately, this may result in us having to cut that feature for the Wii U version, although we're already attempting to find a way to patch it back in post-release! But since it's a major reason why it's taken the Wii U version longer to get through, we had to make a difficult decision on whether to delay the game further, or just try to work it in as a patch so that people can still play the game sooner.
Here's the good news: it's still happening. It might be coming in Summer 2016 instead of Spring now, but we're still making it our biggest priority. I'm a Wii U owner and lifelong Nintendo fan, and I know the sting of companies (or Kickstarters!) canceling the Nintendo version out of the blue. We're not going to do that to you! The Wii U will have Tadpole Treble. I didn't come this far to not have my game on a Nintendo system, by gum! WE'RE MAKING IT HAPPEN. NO MATTER HOW MUCH IT DETERIORATES OUR SANITY!!
Did you back this game in the hope of playing it on Wii U? How do you feel about waiting a little longer?