The Wii U failed at everything but first party support really. The OS was slow and clunky and would freeze, even at version 5.5 it's garbage. It's friends list and online isn't on par with the others. No party chat, the Eshop is trash with no shopping cart and it's slow. The specs were awful, they somehow released a weaker cpu than the 8 year old 360 in 2012.
The marketing was atrocious, they didn't differentiate it enough from the Wii, and the Wii U was such a stupid name, that many thought it was just a tablet controller add-on. Simply calling it the Wii 2 would have even saved some of those headaches.
And the lack of an internal hdd was a joke, not everyone wants to buy an AC powered external HDD to store their games on.
The gamepad was an expensive mess with a cheap resistive screen, was too big and didn't really enhance games at all or changed the way we played.
It was a small box with low power draw (the PS4 used 250w peak on the launch day model and most people seem fine with that) and advertised off tv play, all things that cater to a Japanese market where houses are smaller, and most houses have one or two televisions. It was never going to work out for the west and the west wasn't taken into consideration at all in its design.
Edit:
And let's not forget the embarrassing re-reveal at E3 2012 where people thought they'd blow the lid off all those secret projects they were working on during their year of radio silence. We ended up getting nothing but Reggie fucking around with some Zombi face changing demo, and lots of NintendoLand and Reggie spinning Arkham City Armored Edition as "not the same game, not the same content".
And the Wii U was barren for year 1 because it lacked third party support, and it's other major device, the 3DS, was floundering because it was another failed gimmick that didn't catch on, was overpriced, and also not getting too much third party support.
Al they went full panic mode and pumped out incredible titles in 2012-2013 for the 3DS and cut the price in early 2011 to save that clusterfuck. It's now doing ok.
Then by the time they went back to the Wii U it was essentially dead, with Mario Kart, Smash and Amiibo providing tiny boosts while DKCTF failed to sell over a million copies after its predecessor sold over 6M on Wii. They tried to fill in the gaps by partnering with other studios and picking up dead projects like Bayonetta 2 and Devil's Third, TW101, SMTxFE All sales bombs. They're expensive Xenoblade investment also bombed and got no marketing in the west.
Then a star fox tech demo was shown behind doors, still looks rough around the edges today, Zelda was a no show at E3 2014 and then delayed indefinitely in 2015, when it was all but confirmed the Wii U couldn't be saved and the DeNa NX name drop pretty much meant the end. All major projects after that likely got switched to NX or at least are getting NX ports as the primary focus.
