Here's Slightly Mad Studio's Ian Bell on the issue back in May:
We could reach a fairly solid 30FPS [on Wii U] but it might take a hell of a lot of work. On the other hand, about halfway through us finishing, Nintendo might announce a new console (I have zero knowledge on this BTW but I've heard 'rumours'). Our work might just be the best thing that ever hit that new console in the driving sim genre.
In light of that, I'm not sure I buy that Wii U is too weak to run the game. Rather, it sounds like the team could
try to get it to work but was playing chicken with Nintendo announcing NX which obviously hasn't happened. Slightly Mad Studios probably doesn't think it's worth the effort when Wii U's successor will be announced in less than a year. That is certainly the team's prerogative, but let's not pretend that studio boss, Ian Bell, didn't basically admit as much. Coupled with the fact that Slightly Mad Studios already announced a new crowdfunding campaign for a sequel about a month ago, and it isn't that difficult to see why it wouldn't want to put the time and energy into a late port to a struggling platform when it can refocus its efforts on a sequel that can be ported to said platform's successor. Slightly Mad Studios isn't a large company. It has to do what's best for business. A Wii U port isn't, and blaming a struggling platform is better damage control than "We don't feel like it because we're already working on a sequel."