It was a flop because the marketing and lack there of, the "Wii" name left a big stink in gamers minds, people thought it was a add-on to the original Wii, big game droughts, a lot of the 1st party titles lacked ambition, and overall it just lacked that unique Nintendo idea/magic. The asymmetric gameplay/dual screen gimmick was rarely used and when it was it was terrible and too late (Star Fox Zero).
The Wii U I think years from now no one will remember and look back fondly on it like they do let's say the Gamecube. Almost all the big 1st party exclusives Wii U have been ported or will have been ported to 3DS and Switch. Hyrule Warriors, Pokken Tournament, Super Mario Maker, Mario Kart 8, Breath of the Wild, Yoshi's Wooly World, and Smash Bros all have 3DS/Switch counterparts so the Wii U looks terrible from a 1st party line-up. No one in the future is gonna whip out or buy a used Wii U to play such classics as Amiibo Festival, Mario Party 10, Star Fox Zero, Paper Mario: Color Splash, Mario Tennis: Ultra Smash, Kirby Rainbow Curse, etc.