The Wii U had a lot of little problems that ended up compounding, such as lack of third party support, lack of marketing, and a lack of supporting the gamepad.
I'd say a good amount of problems of the Wii U revolve around the gamepad. It was supposed to be the system's gimmick, like the Wii Remotes for the original Wii, yet Nintendo has barely done any interesting things with it, athird parties certainly didn't use it in innovative ways, and it rose the price of the system for no good reason.
The gamepad just didn't catch on like the Wii motion controls, and was pretty much a result of Nintendo trying to directly succeed the Wii.
"Just for comparison Uncharted 4 was 20x bigger than Splatoon 2. This shows the huge difference between Sony's first-party games and Nintendo's first-party games."
