The marketing for it was terrible, the first announcement it wasn't clear if it was a new console or just a new accessory for the Wii, and since the announcement put way too much emphasis on "New controller" rather than "Wii 2", I as well as many other assumed it was going to be a new accessory for the Wii
It also had a lot of issues in the game departments, it had not many significant titles during most of its run, New Super Mario Bros. U (the 4th New Super Mario Bros. in the last 10 years), Zombie U and Nintendo Land being the Wii Sport equivalent, by focusing on showing off the Gamepad gimmick, but unlike Wii Sport, they did not come with the console for free...
Everything else at launch had been avaliable in other consoles for a while or had little promotion beforehand
Shortly after other games were released, being Game and Wario (the worst Wario Ware imo), Pikmin 3 (only in NoA), Wonderful 101 and Bayonetta 2. While the catalog on the Wii U has improved sort off, is still pretty bland compared to prior consoles and the switch, which is about 2 years old. Look who there is not a single Pokemon game aside from the worst Rumble yet, no Zelda game until BotW (and Warriors I guess), the only Kirby game was the terrible sequel to the terrible paintbrush Kirby on the DS, there was no Fire Emblem (Mirage of Season doesn't count... as a game), no Metroid at all, Donkey Kong Country, Mario Kart and the new IP Splatoon came way too late, and more importantly most of the AAA games being released in every console and PC wasn't available on Wii U
The reasons of this last one are still vague, but the biggest one seems to be that Nintendo force the usage of the Gamepad on the developers, which they eventually just gave up on the extra effort to put a game on a console that was barely selling. And Nintendo also forced the gamepad gimmick onto themselves, ruining many games gameplay like Paper Mario and Star Fox Zero
The Wii U has a chain of terrible decisions and it pretty much killed itself