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.Last edited by The_Liquid_Laser - on 06 July 2021