I think the reason that kh3 is not coming to wii-u has nothing to do with the graphics, but with the horrible sales wii-u has been having. Deus Ex special edition was supposed to be an exclusive until Wii-u started selling poorly, now its everywhere. This holiday for the wii-u will determine whether this game will come to this console or not.
We can also go the optimistic route and say that Nintendo refuse to announce KH3 and told Square Enix likewise until it's within a year's release date.It's still in development, anything can happen.
Either way, I'm still getting a PS4 with my Wii-U .