There is no Shortage of Reasons

1. Architecture/Hardware - The Wii U wasn't necessarily weak, but it had different strengths and weaknesses compared to even the PS360 duo. The way I understand it is that the Wii U was a step up in terms of GPU and RAM, but was fairly different from standard PC or Mobile (though this was largely irrelevant in 2012) architecture, with a fairly slow CPU. That's a major barrier to easy third party support.
2. Branding - It was always unintuitive to casual audiences that the Wii U was more than just a peripheral, especially after 6 years of peripherals using the convention of "Wii Something."
3. eShop - The Nintendo eShop was fairly weak on the Wii U, with clunky backwards compatibility with WiiWare and the Wii Virtual Console, the new Virtual Console coming months after launch and not delivering as many games as the Wii counterpart did despite better tech, and the Wii U's paltry storage conspiring against downloading larger games anyway.
4. GamePad - The GamePad, a fairly chunky piece of tech, featured a 480p screen with 158 pixels per inch. For comparison, the iPad 3 came out in 2012, was similarly sized, had a larger screen, had 264 pixels per inch, and could work more than 10 meters from a Wii U console. It wasn't hard to make an unflattering comparison.
5. Gimmick - The GamePad was a lot harder to sell as a gimmick than motion controls, especially since the controller itself was a lot less visually appealing in 2012 than the Wiimote was in 2006. There were eventually games that could sell the GamePad, most notably Super Mario Maker. But that came out way too late. Also didn't help that the console was limited to a single GamePad, conflicting with Nintendo's usual strength in local multiplayer.
6. HD Leap - Nintendo has historically had trouble developing games after new major tech leaps, and the HD transition was no different. The Wii U had games taking longer to develop and being released less frequently.
7. Launch Lineup - Although there were plenty of games at launch, good games at that, there was no game that made people want to go out and get a Wii U. Nintendo Land, though fun, was no Wii Sports, and New Super Mario Bros U was seemingly the same game you could play on any Nintendo platform.
8. Marketing - This one is a bit hard to quantify, but I feel Wii U ads never really did a good job selling the console in question. The only exception I can think of is some of the Smash Bros for Wii U commercials selling 8 player Smash pretty well.
9. MIA Franchises - The Wii U never had a main series Pokemon game, true successor to Wii Sports, Animal Crossing, Nintendogs, or Legend of Zelda (Breath of the Wild's dual release doesn't count). Other series like Kirby, Fire Emblem, and Metroid are understandable, but those are big franchises to skip out on.
10. Price History - After the $50 pseudo-cut in 2013, the Wii U never became cheaper. These are rarer in recent years, but at one point you could get a PS4 for the same price as a Wii U.

I could find more reasons, but 10 is enough.

The Wii U failed because it was a terrible device. Simple as that to be honest. People knew exactly what it was but the controller was ugly and it was overpriced. Lack of two player anyone?
Come on it was dead before arrival. I understand the concept as to why they thought it would work but they misfired on this one

Wii was a fad based trend in a lot of way, a lot of its audience didn't understand the concept of even iterating upon it like a traditional console (ie: someone releasing the Macarena 2 or Gangnan Style 2).

It would have failed even without the tablet screen IMO. Look at also Kinect was red hot for the XBox 360 and a few years later no one cared about Kinect on XBox One.

The other problem was when they started development on the system, probably a lot of the tablet stuff seemed novel, like being able to use it like a TV remote, off screen, check sports scores, play Netflix, make video calls, in the living room etc. but they likely didn't realize Apple was going to release the iPad which would become a massive success at around the same time and basically make the Wii U look like an also ran.

Then thirdly I guess they thought more mini-game collections (Nintendo Land, which is actually a pretty OK minigame collection as far as that goes) and more 2D Mario would sell the system, but after like 500 minigame titles on the Wii 1 and multiple 2D Mario games that looked similar (NSMB DS, NSMB Wii, NSMB2 3DS, NSMBU), the appeal of that had worn off.

In hindsight their better option would've been to just make a traditional game console successor to compete against PS4/XB1 and then released maybe a "remixed" cheap version of the Wii with HD support and more fitness oriented games and sell that for like $150 for casuals who wanted that.

Havn't we had this thread like a 100 times already?

1) price to performance was poor.
2) it launched at the tail end of the PS3/XB360, and barely ran those multiplat games (PS3 gen) any better at all. Ei. it felt a gen behinde to most people.
3) confuseing marketing (too close to wii accessory level's of confusion, with names Instead of just useing a clear 1,2,3,4,5 ect number scheme).
4) PS4/Xb1 were right around the corner, and looked to make the Wii U look dated by compairison.

Its saveing grace was it had some good 1st party games.
However Nintendo saw the writeing on the wall, and basically put all efforts into next gen... so there were dry periodes without many/any good new games for it.
Nintendo gambled on the future, and basically left it to die. It payed off big time.

Sometimes a bad hardware design is all it takes to kill a console.
Horrible marketing and names, doesn't help either though.

I feel like to many nintendo fans overlook this part.
It was bad hardware design.  (the gamepad never really hit it off, with use.... it was a expensive gimmick)

Then we get threads like this one.... trying to justify other aspects.
While its true it wasn't just 1 thing, but many, a large part of that was just the hardware design.

The Switch is currently two or more gen's behinde in terms of power (compaired to PS5/XSX).
And honestly, because of the handheld aspect, alot of that is just looked through the fingers with.

However the Wii U, was just a traditional console, and it was showing its age, already when it launched.
(Basically launched right before PS4/XB1, and was a gen old and dated.... too early on)

1. Tough to develop for. A console's first-party games have to pave the way for success before taking the risk of developing for annoying architecture. There are tons of game that could've run on Wii U that never showed up because of the poor sales.
2. Marketing and branding. The marketing for Wii U was often childish or confusing. And the Wii brand had mostly run its course from 2011 and on.
3. Price. Why pay $300-$350 for a platform with small internal storage and comparable power to PS3 and 360 if there aren't killer apps?
4. Lack of software that attracted the audience. This goes hand in hand with some of the other points.

Awful hardware, awful advertising, awful and confusing name.

Sub-par third party support

Pretty much everything opposite of what the Switch is.

There's a lot of reasons, but I think the one that is overlooked is just how bad the games lineup was.

It launched with NSMBU as its big title, and that was a fun but pretty safe sequel. Then, there was kind of nothing major for about a year. Not only did the library fail to take advantage of the hardware, but for the most part, they weren't very exciting titles. They're fine games for the most part, which is why they're selling on the Switch, but very few system sellers. You likely already had similar experiences on the Wii or 3DS, so it was hard to justify buying a Wii U, particularly with its other flaws. There were also some straight up bizarre choices made, like whatever the fuck they were doing with Wii Sports Club. While releasing a (very good) version of Smash on 3DS was great for gamers, not so much for selling Wii Us.

The Wii U evendually did get some genuinely good and novel games, like Mario Maker, Splatoon, and Breath of the Wild, and the first two at least actually sort of showed why the Gamepad was a good idea. I still wish I could use the gamepad for Splatoon 3 and Mario Maker 2. But by the time those kinds of games came out, it was too late.

I would love to see an alternate past where the Wii U's launch title was more like the Switch. Like, BOTW and Mario Kart 8 were available at launch, and the first year had Pokken, Mario Maker, Xenoblade Chronicles X, and Splatoon. I wonder if that would have been enough to kind of salvage the Wii U and have it sell at like XBox One levels. Sadly the world will never know.

I still think there's something to the asynchronous dual screen set up. I wouldn't be shocked if it's something that the Switch 2 could do. Nintendo does like to try and rejigger their failed ideas, and sometimes it works out.

The casuals were confused and didn't buy it, there is no big reason aside from that. As a hardcore gamer who kept up with gaming news, I was confused until after it had released. At it's most simple, it failed due to confusion.

It failed because where the Wii removed barriers to entry for gaming, the Wii U added them.

The Wiimote made gaming simple, accessible, and fun. The Wii U Gamepad made it cumbersome and convoluted.

It was a gimmick nobody asked for, it added to the price of the system, and the fact only one was supported per console complicated multiplayer.