I personally think it had a marketing problem. although we all know the game pad was a massive failure, I mean you literally couldn’t get a replacement or an extra. Not to mention the name, a name like Wii U just screams upgrade and not new console. And the backwards compatibility was a mixed bag from what I have heard also the capability’s were pretty bad plus it didn’t have a new Zelda game for a really long time and was just a shit switch lite. But was it a bad console?
It wasn't bad at all. I wish it would have sold better, cause the hardware was good. It got some heavy hitters released for it, but the lack of sales early on kind of doomed it, because 3rd parties didn't want to support it, which meant the library was lacking and it continued to not sell well.
I guess it sold poorly for 2 main reasons. Many people didn't like the gamepad. And games generally didn't take advantage of it or took poor advantage of it, which reinforced that belief that the gamepad is trash. Nintendo's marketing was so heavily focused around the gamepad, but that might've harmed more than it helped because they just didn't have enough to show for it to convince people that it was anything more than a pointless gimmick. Nintendo heavily marketing the gimmicky aspect that nobody liked then made people dislike the console itself. It didn't help that you needed to use the gamepad for certain things, hell, you needed it just to change the settings (wtf Nintendo, seriously)
Everyone I know preferred to use a Wii U Pro Controller. I personally liked using the gamepad, but the battery drained in days/weeks even when turned off so it was always dead when I wanted to play, That turned me off it a bit.
If the gamepad was optional (and wasn't a forced purchase inflating the price of the console) except for games that actually took advantage of it in unique and cool ways, like Nintendoland, and wasn't forced in games like Star Fox Zero where it was more of a hindrance than a help, or to do something as basic as changing the system settings, maybe the Wii U would've been a success.