The controller. I can see why in Nintendo's internal logic they came around to designing such a thing, but it just doesn't work in the current world. It's basically a clunky tablet, but amazingly enough, people are already overflowing with REAL tablets whether as their phone, actual 'tablet' format computers, whatever. So paying extra for such a thing is not seen as a value added proposition. Game-design wise, it just doesn't add much, if you are looking at the tablet screen you aren't looking at the TV screen, so it isn't adding information, the same information could be presented in modal screen over lays. A real game controller that offered 6axis style controls, force feedback on controls/triggers and enhanced rumble like XBone would be a great starting point. And imagine the 3rd party games that could have been sponsored for the development+sale cost of this over-bloated tablet controller.

Otherwise, the console architecture an uninspired extension of last gen, it has something like 4 major memory pathways and in continuing with Power architecture, Wii U is just flat out more difficult to program for and offers less pay off even for trying.

Twilight Princess on Wii sold more than Wind Waker and Twilight Princess GC combined.

DKCR Wii has sold more than four times as much as Donkey Kong Jungle Beat on GC.

I think this is also another good point in relation to the casual audience, although I don't think it's the primary reason.  The Wii remote was so simple and accessible, yet powerful.  The Wii U's controller is just a mess of a ton of inputs.  It's busy and can definitely appear daunting to those that don't play games often. 

Marketing.
It has 3D World, Pikmin 3, Wonderful 101, Sonic Lost World, NSLU, Game and Wario, Monster Hunter 3 Ultimate, Lego City Undercover, NSMBU, Nintendo Land, Zombi U, etc. That's enough games to buy a console. They really started marketing almost a year after it came out.

Price wouldn't matter because It's the least expensive, the concept isn't bad, it has good games, and PS4 and Xbox One impossible to find,

Wow, I kept hearing about gamecube having killer software sales and never took the time to look, highest game was 7m.

Several things.

Marketing is awful
Name is awful. Like seriously the worst console name I've ever seen
Lack of games
High price
underpowered

the same reason GC didnt take off

nintendo console sales have been on a downward slide after nes, many people got mislead with the wiis performance thinking that nintendo has magically become the king in the video game business. when what saw with the wii was what many sales forecasters like myself where at the time (2008) calling a fad it just took 2-3 more yrs for it to be seen in the data.

i just think the main problem is that nintendo has failed to evolve they have just stuck to the kiddie image and franchises for over 20 yrs now. gamers like myself are always looking for the next new thing, in the end its just a game, games and gameplay over time become tiresome and boring if nothing in the game play changes. even if it did get full 3rd party support and had all the mature games that psxb has i dont think it would matter b/c gamers dont view nintendo as that console and the gamers that have a nintendo console dont want to play those games.

Yeah, the Gamecube gets talked up a lot by its fanbase, sometimes to degree that doesn't reflect the reality. ;)

And you were wrong.

The Wii would have been a fad if it fizzled out in 2009. But it didn't; it had a normal console lifespan.

Why it hasn't succeeded? Because the market doesn't need 3 home consoles, two is enough and that market is captured by MS and Sony with true-HD consoles that are capable of pushing the envelope.

Yes, it's that simple, we need and want Nintendo games, not Nintendo consoles. I just wish Nintendo will realize this in my lifetime so I can finally play Mario in true HD.