The only reason Wii sold well was because it was cheap, easy to hack and Zelda/SSB/Mario wheeled in the dough. Other than that, Wii would have lost it were it not for those 3 things.
Nope, the reason the Wii sold well was the Wiimote (and the price of course). At the time, motion-controls were new and innovative. They haven't been used in a serious manner in videogames before and it really had a "cool factor" to it. Even before the Wii launched, everyone wanted to try out the system.
Part of the genius of the Wii was that the barrier to the casual market were broken thanks to the simple control-scheme. Instead of having to press all these buttons on some complicated controller to play golf, just swing the Wii Mote like a golf club! Or to play tennis, just swing it like a tennis racket. That shit was simple and easy to understand.
It even garnered a significant amount of interest from the hardcore in the beginning as well. I mean, using the Wii Mote as a gun in Metroid Prime 3 was pretty fucking cool at the time. As well as being able to actually swing a sword in real-life and have it work in-game (even if that wasn't realized properly until Red Steel 2).
With the Wii U, Nintendo forgot all about the casual market. Instead of the simplistic controls of the Wii, they introduced a controller that had a huge screen and all the traditional controls that scared casuals away in the very beginning. Even worse is that it had a very "been there, done that" feeling since the advent of tablets such as the iPad. If they had launched two years back before or around the time the iPad came out, the Wii U would likely be selling much much better right now.
So the casual market is gone, what about the hardcore? Nintendo hasn't done much there either. They still released a console that pales in comparison to the other next-gens in terms of specs. They get a lot of late ports that everyone has already played and they sell like shit because Nintendo hasn't done much to cultivate a hardcore base on their system. They've failed to properly advertise the benefits of the Wii U's controller with their own games, with Ubisoft even doing a better job than they are with ZombiU.
Right now, Nintendo seems completely lost. They say they want the hardcore but their actions show otherwise. They claim to be trying to reclaim the casual market but seem to forgot how. I'll give them until this holiday season before I put the final nail in the Wii U's coffin but things aren't looking bright right now.