An Atari VCS 2600 is a dedicated gaming device as well.
Wii U's CPU is, in plain words, painfully slow. Now Project CARS has really demanding physics. Basically Wii U cant handle the tire model at all. Just read a little bit in the WMD forum.
So they'd have to change the complete tire model to a much simpler as well as tons of other things. Which means Wii U owners would get one of those 'lazy ports'. In fact the CPU is to slow for a lot of things.
If Nintendo only at least had went for a 1.6GHz PPC quadcore or, better for everyone but Nintendos own devs, an x86 quadcore with 1,6GHz that would be much less of an issue.