A couple previous interviews make this kind of confusing. Either they are straight-up lying or were using much more powerful devkits and never testing actual hardware or something.
http://mynintendonews.com/2014/01/07/project-cars-will-showcase-the-hidden-power-of-the-wii-u/
“No, if anything I think it’ll showcase the hidden power that the Wii U has. There’s no equivalent of Project CARS on Wii U, yet we hear the shouts from Nintendo fans loud and clear that they can’t wait to get their hands on this kind of experience so we hope to make them proud with the quality of the visuals.
“Sure there are maybe more stringent technical constraints but the same core experience will remain, plus additional features that take advantage of the controller’s unique second screen and touch interface.”
http://nintendoeverything.com/project-cars-extremely-promising-on-wii-u/
[/quote]I literally just had a blast racing around one of our tracks in the rain (the most graphically-intensive weather setting) and it held up pretty damn well. Of course there are optimisations still to be done but side-by-side, the Wii U version we have running in the studio is extremely promising. [,,,] Every screenshot or video you’ve seen from Project CARS so far has been taken by an actual player – any one of our WMD community members. So there should be trust already that what you see on websites or on YouTube is actually what you’re going to see in the game. There are no plans to abuse that trust when it comes to the Wii U version.[/quote]
Speculation here, but maybe they made a game in Unity for Xbone/PS4/PC, then lied in a bunch of interviews about the Wii U version being worked on. After releasing it on PC/PS4/Xbone, they started by porting it to Wii U and found that on low settings, it gets 23 FPS. They probably just don't want to put any real money or time into it.
Even if they weren't lying back then, it looks like they were now.