Like Kimishima said, certain people at the top within Nintendo thought that because Wii did so well, they could replicate that with Wii U and achieve similar success. He didn't agree and said he thought Wii U would need to be different and not simply do the same thing again. But more than that, the Wii U concept came together around 2008, at a time when iPads and tablets were the latest trend, and everyone was buying one for themselves and for their kids. Nintendo saw that and thought, "How can we do something that uses a tablet interface, but apply it in a unique way and make it "Nintendo-like", and they came up with the GamePad idea. We saw how that worked out. By the time Wii U actually came to market, tablets and iPads were onto their third generation and had become standardized and ubiquitous, and Nintendo completely missed the boat.
In other forums and years ago, I've often said that I think Wii U would have been at least a modest success and perhaps at least out-performed the GameCube had they just released the console by itself, bundled just with a Pro Controller, for $199, and only sold the GamePad seperately. But even then, they lacked a killer launch title to motivate people early on to buy the console. So without Smash or even Mario 3D World or Pikmin 3 being ready for holiday 2012, it never had a chance honestly. And once you've lost momentum, it is nearly impossible to get it back. The only other chance would've been delaying it to holiday 2013, and having Pikmin 3, 3D World & NSMBU all ready for Nov/Dec back-to-back-to-back. Because 2014 was the Wii U's best year of first-party output, but the console had already died commercially as of the year before. So there was nothing else they could do to bring it back to life.