Its definitely a 2 part problem for the wii u IMO.
I have one, got it day 1, and i LOVE it, ive bought quite a few games for it and spent lots of time with it. Unfortunately, because 3rd party mostly seems to care about pushing graphics to the max, which is not possible on Wii U unless you dedicate the game engine/ development to it, and making multiplat games.
it really is a shame because if there were more games to take advantage of the asymmetrical gameplay options available through use of gamepad + other controllers, people would see it truly is a good system, just misunderstood.
In fact, one of the only games that showcase this in my opinion is Zombiu, which was an amazing game that got shit reviews because it was compared to other faster paced zombie games. my friend tried zombiu multiplayer the other day at my house and said he wanted to buy one when he got his tax money just so he can play the multiplayer with his brother and sister.
if more people take advantage of the asymmetrical aspect of it, there could be some truly amazing games this gen.
and also, for the people comparing sales to Wii sales, there really isnt much of a reason to compare it, the Wii was a completely different monster. The wii was the first game console to completely "revolutionize" how games are played, focusing on the motion controls first and foremost, and also it was cheaper than the competitors and came with a game to showcase how motion controls work. THATS why it was a hit with the casual crowd, the same people who probably saw it and went "wow, I can play motion controlled sports games in my living room with people, thats amazing!"
thinking that all of those people would come and buy a Wii U is like catching lightning in a bottle, it simply doesnt happen.
and before people think of this as a pro-nintendo fanboy rant, I have a Wii U, a PS4 and a PS3. I used X360 most last gen, and i have a gaming PC (albeit out of date). I play games to play games, not for the companies.
I just dont get the hate for the Wii u :/