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.