I predicted the Wii U would fail (but not to the degree it did), and the reason was simple:
it didn't appeal to anyone. The Wii appealed to Nintendo fans and non-gamers. It was simple, easy to use, and fun. The Wii U was big, clunky, and not fun. The Wii had great Nintendo games like Twilight Princess, Brawl, and Galaxy, etc. etc. The Wii U didn't even get an original Zelda game (well technically it did but c'mon) and 3D World was almost an insult (I love the game, but damn I felt literal embarrassment when I first saw the trailer).
Nintendo banked on the tablet phenomenon, but the problem is the controller was never utilized properly. With the Wii there was Wii Sports. With WII U there was what?
Overall, it was always going to fail. It just appealed to no one.