The main problem is the system doesn't have a whole lot going for it other than Nintendo franchises which I think still only gets you between 18-29 million. The Wii brought in other consumers with the novelty of motion gaming, but that has been mined dry too, so Wii Sports/Fit/Party are not moving the needle on Wii U sales at all.
The other issue with "Nintendo franchises" is the ones that actually are capable of selling hardware have kinda become fewer in number as Nintendo chased casuals the last 5-6 years.
It used to be in the SNES/N64 days, something like Star Fox or Wave Race or Killer Instinct or F-Zero could be 1-3+ million sellers, but today it's basically become the Mario platformers + Mario Kart + Smash Bros. + Zelda (and even Zelda, if its not a realistic Zelda it's not a system mover). Then you kinda have DKC which is a decent seller still (I think) but probably redudant in the audience with Mario already there.
Everything else really is not moving systems (as much as I like X and Bayonetta 2, those are probably going to sell as well as Pikmin 3 being optimistic).
