Nintendo had an off development cycle. They did everything wrong with the system, except for their games. I love the effort they put into it but it doesn't warrant a $300 price tag. The controller killed them the most. That $180 used for the pad could have been used for improved specs and storage. The system is just in a land of its own, Nintendo Land that is.

Nintendo messing up big time on marketing, a very stupid name (a part of marketing, but still), no third-party support, far too expensive for the hardware, etc.

Even the Gamecube had better third-party support, the only reason it did bad was because it had to face the juggernaut that was the PS2 and it had no DVD support like the other consoles.

The WiiU's bad pricing, hardware, release schedule, and marketing all play a role. I don't think Nintendo could have done much more to stop the WiiU succeeding :p It's a shame too, because many of its titles are excellent.

They might have been able to get away with the pricing and hardware had they properly justified the gamepad, but we're 2+ years in and there are very few games that scream "this controller is a necessity", something the WiiMote managed from the start.

The marketing (and third party selection) on the GCN was better by far. Still, these two represent my favorite Nintendo systems outside of the SNES. Too bad the rest of the world will find out about the Wii U's killer library after the fact, much like the 'Cube.

A lot of things. The marketing is a major factor though. Gamecube's was underwhelming, but I feel the Wii U's actively pushed people away.

Nintendo systems have been on the downhill ever since the PS came into existence.

Marketing and price are probably the two biggest factors.

I think the fact that wii u is underpower compared to other consoles on the gen is a reason why 3rd party games don't sell well on wii u and lack of 3rd party support mean the console will fail

Well the gamecube was better in almost every regard, better marketed whilst being more affordable at the time. I ticked all of the above but kids wanted a PS2 not a gamecube, so I don't agree with that point but everything else applies.

Essentially the only basis on which the Wii U would be selling better is that its following on the success of the Wii but I guess they didn't actually keep any of that momentum and damaged their image with the core.

more games, more 3rd party support, lower price, better name
in short, pretty much everything.