It was weak AND expensive. A home console that checks both those criteria has a steep climb ahead of it and had better crush the competition with software--which the Wii U did not.
You can have your gimmicks all you want but if they drive up the cost of the product by a significant degree then they need to be absolutely amazing.
Relative to that, it was also a gimmick that was welded into the Wii U. Had they been able to divorce themselves from it they way the Xbox One did with Kinect then the Wii U might have been able to pull some decent numbers as a low price option.
And that's just off the starting line, never mind all the other missteps Nintendo made with the Wii U.