A big part of what made it fail was this:
Marketing was the biggest problem. Do you think PlayStation would still rule the market if it was called the Pee U?
to this day people are more confortable with things getting named 1,2,3,4,5 and the memes with and ambuiance doing a Wii-u, Wii-u sound were common as the name sounded ridiculous to lots of people already into gaming.
All down to naming and marketing. I mean, not all, of course... it still wouldn't have sold well if it had a better name and better marketing, but probably twice as many as it sold, at least.
People weren't bothered by how good or bad the UI was, or the loading times, or the Gamepad, or even the games. The console was unappealing from the get-go due to awful marketing and naming, thus making it not sell enough for anyone to judge it on those terms. Someone who doesn't buy a console because of its negative aspects is someone who knows something about the product, and if they know about it it's because they have some experience with it. But people didn't have any experience with the Wii U, because no one had one, because it was marketed like shit.
Honestly, if you look at what the console does wrong, I'd argue the Switch has as many issues, if not more. But it has an appealing concept, a snappy name, and great marketing. Then it had a groundbreaking game as a launch title. But if the marketing was wrong, everyone would've ridiculed it for having no games - take away BotW and there was basically nothing at launch. But because of the hype they generated with good marketing and a good game, it got momentum, and then people continued to like it for its positives, rather than its many negatives. Of course, it being a hybrid console helps naturally, as it's just a good concept.
And this also, people not that into gaming thought that the console was merely and add-on for the original wii
Gamepad should've been peripheral (with much better range), so that CPU/GPU/RAM can be much beefier.
Lots of people in my university that were into gaming weren't willing to buy it for a few reasons inlcuding this, they wanted the gamepad to be able to be a portable console on it's own, they didn't have any problem with it being a tablet-like controller but to them it wasn't justified that the control had that form if it wasn't possible for you to use it as a portable console or a tablet, or at least to have more than 10 meters in your house, people wanted to use that curtain mode to play on the gamepad on their own houses while other family members used the TV, since it wasn't common here for families to have more than one TV, but it wasn't possible as the controller range was very short, also while the original wii here did cost roughly the 250 or so dollars from its launch, from the changes in our economies and other things the Wii-U here went for the equivalent of 500 or more dollars, people weren't going to invest in the Wii-U while their phones were portable, and you could game with them on the go while also being able to do a lot of other things.
And finally i should add that people acquire strange customs in gaming, the dudes from university aside from the relation of price and the poor benefits they perceived from the gamepad, while showing interest in the games, because yeah they said they wanted to play Smash Bros, Mario Kart, Xenoblade X, Super Mario 3D world, Bayonetta 2, Wind Waker HD, Twillight Princess HD, Super Mario Maker, Yoshi, Donkey kong, Rayman and other games over the years, were always saying that they weren't going to buy the console despite liking those games because since there wasn't online chat they couldn't insult or swear with others, and that since there wasn't some kind of "achievements" they also couldn't brag or show off, then they preferred to play other games on Xbox or PS even if they weren't good games, only to accumulate points in their "gaming accounts".