The fact it only had one real controller and focused primarily on single-player experiences. That broke the Wii U.
There was not really any way for it to have a killer app because the hardware prevented it.
Mario Kart 8 is a giant mess on Wii U for multiplayer. Meanwhile, it's a tremendous experience on Switch, largely thanks to the fact that you can take it out with you and play large local multiplayer games, and hook up online to fill the empty slots with other players.
As has been said before, the Wii U was more of a sequel to the Gamecube than the Wii. Similarly, the Switch is more of a proper sequel to the Wii.
I describe myself as a little dose of toxic masculinity.