I do not agree that the Wii U had a poor first party software library, however, it took far too long for the Wii U to build a decent library and by the time it was built the console was already a long lost cause. There was a massive drought for the first year on the market where the Wii U had almost nothing except for New Super Mario Bros U. Nintendo dev's were openly admitting that they were having a lot of trouble developing games at 720p and so the reason for this drought was something that was quite embarrassing for the company and probably a big cause of concern for its shareholders. Furthermore, the 3rd party library was almost non-existent, which was another significant problem for the console.

It is no doubt though that the hardware was a factor in the Wii U's failure, but it is not the #1 reason in my opinion. Nintendo confused a lot of people by re-using the Wii name and showing people holding Wii-motes in the commercials where people were thinking that the Wii U was an add-on for the Wii. Also, the Wii U did not have a killer app at launch to show people how to make use of the game pad which is a big part of what made the hardware such a turn-off. If the Wii U had launched with a game like Super Mario Maker that clearly showed developers what could be done with the gamepad as well as clear marketing, I think that the console could have sold twice as well as it did even with the clunky gamepad.

That said, the gamepad could have easily been slimmed down, but the Wii U launch was such a catastrophe for the reasons mentioned above that Nintendo did not even see it worthwhile to try to revise the hardware or give a significant price-cut. By late 2014, Miyamoto was already teasing about new hardware which means that Nintendo probably made the decision to kill the Wii U well under 2 years into its life. The games that came after this point were just to satisfy the base that bought the console, Nintendo had no expectation that they would change the Wii U's fate. In fact, I believe that Nintendo released games like Smash, Splatoon, Mario Maker and maybe even Mario Kart with the expectation that they would get re-released on the Switch in order to make the investment in the game's development profitable for the company because the Wii U's paltry base was not enough to accomplish this by itself.

Wii U exclusives were pretty great overall with some exceptions like Amiibo Festival or Starfox Zero.

A big part of what made it fail was this:

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.

And this also, people not that into gaming thought that the console was merely and add-on for the original wii

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".

There's a difference between great games and system selling games. For instance, NSMBU, the big launch title for the Wii U. It was a fine Mario platformer, and a great overall game, but it wasn't doing anything exciting enough to entice people to want to buy a new console for it. 

The Wii U's first couple of years were very sparse, especially the first year where there wasn't anything major from launch until Pikmin 3 (which itself isn't a huge title), a 9 month gap. By the time the Switch actually started getting some more ambitious titles the system had already been labeled a failure and it was too late.

The only thing I don't like about the U's hardware is the glossy and curvy finish of the console, all in all almost looks like a Fisher Price toy, including the Gamepad. I also dislike the fact that it wasn't able to support lots of PS4/Juan games.

Okay, I thought of a hardware problem no one’s mentioned (I think). This is a big BIG issue to me but I haven’t heard a lot of people talk about it so maybe it’s just me:

It only had ONE gamepad! How the heck is that fair to your friends. It would have been cool as heck if each friend could play on their own private screen. I get why they didn’t do it, but it made the local multiplayer dynamic lopsided.

Smash, Splatoon, and Mario Maker didn't get re-released on Switch though, they got sequels instead.

I am sure that we can admit that there are a ton of re-used assets in these games from the Wii U, though.  I forgot to mention BotW in that list too.  I remember Sakurai say how he could never go through the stress of building another Smash game from scratch like he did on the Wii U.  Most Switch owners never owned a Wii U and we got a better game at the end by being able to build off of the assets in the Wii U installment.  It was a win-win for Switch gamers but it destroyed the Wii U's legacy as it basically has no unique experiences anymore.

I know this and also NSMB games are trash. Awful games.

Smash on Switch is a new game. New engine. New lighting. New models. It's a new game.