There's plenty of theories out there as to why the WiiU didn't do as well as other consoles. I have my own, which centers around the Switch's eventual release, and what Nintendo wanted to make vs. what they could achieve at the time. My guess is that the dream of making the Switch was very alive at the time, but the hardware was not quite ready. A pivot to still incorporate the design DNA, but also save on iteration costs was likely made to keep development cheaper, hence the second revision of the CPU and GPU platform that we saw from the Wii and the GameCube before it. Because this stopgap console wouldn't be the new design they wanted, and they were iterating on the Wii platform, there was a decision to call the new console the "WiiU". This smells of "makes sense in the engineering department, but not to marketing".
As a result, die hards (admittedly, like myself) were happy to purchase the console, and were willing to get read up on the differences, but capturing the casual, and probably more importantly, the parent audience, who remarkably during the NES -> SNES transition were adamant about not purchasing a new console because "they already have one" didn't understand that, while there was backwards compatibility, the console was certainly capable of standing on its own. It would have offered neat experiences with games like Breath of the Wild taking advantage of the WiiU gamepad with screen, and otherwise been easy to write software for as an existing developer on the previous platforms. It would have been a quite fine stopgap release if it had been marketed appropriately.