As to what happened, the team wrote the following:
“We had information about our engine, two years back, when we announced support for the platform. This had assured us we could port to the Wii U with relatively little hassle by now. However, due to ongoing internal contractual issues beyond our control between the platform holder and engine developer, we just don’t know when or if this situation will have a good resolution. We are looking into the options available and doing what we can to help push the issue, including posting about it here.”
Surveys to be sent out to Hyper Light Drifter’s Kickstarter backers will still include the option to receive the Wii U version. However, in the case that Hyper Light Drifter doesn’t come to Wii U, the team says it will offer a key for another platform.
Oh, and the image you can see at the top of this article? That’s the near-final cover art for the game.