I really enjoyed the handful of games the Wii U got, but so many things went wrong.

Marketing was awful, as others have noted.

The Gamepad had a lot of limitations. Bringing the DS's second screen to a console seemed like a good idea at the time, but it just couldn't be used the same way. Human eyes cannot focus on two screens at different distances from the eyeball, so you had to shift your focus to one or the other. Multiplats couldn't be designed around it when other platforms had a single screen, so if they shoehorned in anything for the Wii U port it was obviously an afterthought.

Ironically though, I think the Gamepad's best feature ended up being its biggest double-edged sword. In order to support Off-TV Play, games had to be fully playable on a single screen again, disincentivizing developers from being too creative.

Third parties abandoned it pretty quickly. Marketshare wasn't there, specs were behind, and the gimmick wasn't worth extra work to support. Of course it was mostly a matter of marketshare, because if the system had been flying off the shelves they'd put up with the latter two caveats, but because it wasn't, they wouldn't.

There's also the fact that the rising cost of HD game development kept Nintendo from being able to adequately support both the 3DS and the Wii U at once. Once they lost third parties, Nintendo knew that they just could not keep the Wii U afloat on their own, so they had to make a decision to let it go in order to prioritize the better-selling system, then shift gears to have only a single platform for the next generation.