Problem with the Wii U was the selling point being the gamepad wasn't a must-have gimmick like the Wii & Switch.
The Wii was a must have cause it made it seem like you can have a bowling alley or other sports in your living room using motion controls which were very simple to understand, it truly changed the game and was a gimmick never seen before and it set itself apart from its competitors enough by providing a gaming experience you can't get anywhere else.
The Switch set itself apart by allowing you to have the option to play console like games on the TV and on the go, it's been done before by other companies but it was far less seemless, and doesn't offer the games anywhere close to what the Switch offers. Many brought the Switch for the idea that they're getting essentially 2 consoles in 1 which stood out heavily against its competitors cause of its ability to play console games on the go in a way that fits many people's lifestyle
The Wii U failed to stand out much. The gamepad wasn't a must have feature. The gamepad did some cool things, but none of them made it a must have. For example, playing an adventure game where you'd need a map or inventory often like Zelda with the map & inventory on the Wii U gamepad right there to check without pausing out the main game was cool, but players could easily live without it, only makes checking a map like 2 seconds quicker which isn't worthy enough to spend 300$ on.
Asymmetrical gaming was cool, and for me personally I still really love the Wii U for that feature and made certain games only possible on the Wii U, but again it's not a crazy must have feature since not many games in the first place would use or benefit much from asymmetrical gaming.
Playing off the TV was also cool about the 2nd screen, but it's range was so pathetic to the point where it almost defeats the whole purpose of having it. You couldn't even go to another room with the gamepad without disconnecting from the Wii U, which made that feature only useful when someone else wants to watch TV, but very few times would a scenario like that ever happen. Again, it wasn't at all a must have feature.
All the Wii U gamepad did was add several cool nice conveniences, but nothing game-changing enough to wow people into spending 300+$ on it. A lot of the things the Wii U gamepad did people could live without easily.
With the Wii U gamepad not offering much innovation, what you had left was just an standard Wii that people already had in their living rooms for years and lost interest in the motion controls it offered for an expensive price.