Because the Wii U was a stupid idea implemented even worse. The tablet was effectively tethered to the system and could barely let you move outside visual range of your TV. It made it completely redundant as a concept. The low resolution of the tablet's screen also did not help at all, as well as the washed out colours. It wasn't the power that did this, and the naming fiasco in my opinion is exaggerated. Switch has many other new issues, such as Nintendo seriously dropping the ball for the first time on quality of manufacturing standards (all my Nintendo controllers, bar my first N64 controller still have no drift -- but on Switch I have already witnessed drift on more than 4 out of 8 joycons that I have owned; my SNES and NES controllers that are 25 and 32 years old still work and function perfectly; my first Game Boy from 1989 still works as the first day, if only for slightly washed out screen and muffled sound). Switch is, also, ergonomically inferior (to an already ergonomically horrible Wii U). Ergonomics have generally gone down in quality in recent gens with Nintendo, after the incredible work done during the Gamecube era.

But at least the Switch is not a stupid idea. It is a brilliant idea. It is the best handheld of all time, and allowing you to play it on a TV screen is a fantastic concept that evidently works. It is Wii U in reverse.

Wii U just oozed low quality jank. From the gamepad to the hardware and the software line up, it felt like a Fisher's Price console for toddlers.

I'm still really glad I own one though. It is the ultimate Nintendo box: it plays NES, SNES, N64 and tons of other retro games via VC, it plays Wii games and supports all peripherals, it plays Wii U games (the handful that are worth playing anyway). Considering the Wii is probably one of the best consoles ever made with a huge and amazing library, the BC saved the Wii U from being a total disaster.

The ridiculous Gamepad looked outdated on the day it launched. It reminded me of the Sega Game Gear that was 20 years older.

At the time it launched, tablets were becoming popular. Much larger screens, much smaller bezels, much thinner profiles. It looked ridiculous. Also, it was never well utilized.

I like the Wii U quite a bit, but I knew from the start that it was unlikely to be particularly successful. I bought one because I buy every Nintendo system, because I want to play Nintendo games. I don't regret the purchase. But, I think a lot about how stupid the Nintendo executives had to be to release that thing in 2012.

I will say that in my opinion, Mario Kart 8 is one of the best games Nintendo ever made. I like the controller options on the Wii U version.

That game is the crown Jewel of the Wii u. But, like most of the other good games, Nintendo just retread them for the switch. Those of us loyal Nintendo customers, that was like me that mostly by Nintendo consoles to play Nintendo games, have been kind of screwed with this approach.

Sakurai:

For Ultimate, because the development company is the same, the know-how wasn’t reset – it isn’t like we forgot how to make a good game in between games. There’s no need to change the game’s “architecture” between handheld mode and docked mode, either. We can re-use assets from the previous game, too – things like that.

Source: https://nintendoeverything.com/sakurai-talks-development-of-smash-bros-ultimate-future-and-more/

There is a reason why Nintendo was able to release BotW, Mario Odyssey, Mario Kart, Smash, Splatoon, Mario Maker all within a window of a little over 2 years on the Switch, while these entries took a generation and a half to get rolled out on the Wii U (last few years of Wii + entire Wii U life cycle).  It isn't just because Nintendo got better at developing in HD, Nintendo also finally figured out the art of re-using assets like most of Sony and MS's 3rd parties do on a yearly basis.  This never happened between the GCN to Wii and between the Wii to Wii U because these consoles were so radically different in terms of capabilities and gameplay styles.  I expect that BotW2 will re-use a ton of assets from the first BotW as well.

If you want to know why the Wii U really failed, then you need to understand why the Wii succeeded.  The short version though is that the Wii U was basically a console for no one and that is why almost no one bought it.  Here is the longer version:

Go back to Generation 6.  The PS2, XBox, Gamecube and even Dreamcast are all selling to essentially the same type of people.  They like a controller that is similar to the PS2 controller and they want hardware that is as powerful as possible while still delivering a fairly reasonable price.  When everyone follows this basic model, Playstation is going to win, and they did indeed win by a huge margin in this generation.  That leads us to Generation 7.

In Generation 7, the Wii sold over 100m units.  They did this by appealing to people who were not gamers in Generation 6.  They made the controller simple and made fun games that went with it like Wii Sports.  They also kept the price tag down by making the system significantly weaker than PS/XBox consoles at the time.  All of these things irritated the core gamers from Generation 6.  It even alienated some Gamecube fans even though the Wii had sequels to Gamecube style games like Mario Galaxy and Skyward Sword.  However the Wii also attracted two new groups: 1) People mostly new to gaming who especially liked Wii Sports and/or Wii Fit, and 2) old school gamers who liked games more like NSMB, DKC Returns and the Virtual Console.  They also had a few games like Mario Kart Wii which could reach both of these groups and Gamecube loyalists.

Now we get to Wii U in Generation 8.  Who is this console for?  It has what I would call a PS2/Gamecube style controller with a giant tablet shoved into the middle of it.  It is also significantly weaker than the PS4/XB1 systems of the same generation.  It isn't for the core gamers of Gen 6, because it is too weak.  It isn't for the new gamers on the Wii either, because the controller is too complicated (and it also makes the system ~$100 more expensive).  Old school gamers didn't really go for it, because they didn't really like the 2D Mario game.  Gamecube gamers really didn't go for it either, because they didn't really like the 3D Mario.  It is a console for no one. 

The first Wii U game which fans seemed to universally like was Mario Kart 8 and a few months later there was Smash Bros.  Both of these games released in 2014 which is about 2 years after the Wii U's launch.  By that time it is mostly too late to create momentum for the console.  Gamers weren't talking about it and third party developers had given up on supporting it.  From 2014 and on it had several games that would mostly appeal to Gamecube gamers and maybe some old schoolers.  Even then it only got a few major releases each year, because Nintendo was also developing games for 3DS.  Third parties didn't support it much either, so there were not really games to play in between major releases except for on the Virtual Console.

So, the Wii U did have a few good games on it and these were mostly split between Gamecube fans and old schoolers with neither group really being happy.  Given the lack of third party games this meant it was a terrible system for anyone who wanted to own just one console.  However, if you take the better games on the Wii U and throw them in with everything else on the Switch, then suddenly the games look really good.  They aren't trying to carry a whole system on their own like they were on the Wii U.  Instead, they are a nice bonus on an already popular system with lots of games on it.

This has been discussed a thousand times, and the general consensus is bad marketing, and consumer declining interest in motion controls and associated products. See Wii 2010-12 decline, Wii U sales and xbox kinect decline.

The biggest problem is no killer app early enough in the console's life. Nintendoland is not Wii Sports, and it is not Breath of the Wild.

Yeah go look at Twilight Bridge and tell me it's the same game. It's a new game. It was confirmed a while back it had a new engine. Besides eyes tell you new lighting. Models. Stages are remodeled. End of.

The gap between Switch and Wii U in capabilities is bigger than the gap between Wii and Gamecube.