1- Expensive for what it was/offered
2- The Selling point/gimmick wasn't well implemented (even Nintendo didn't use it very well overall)
3- Bad marketing
4- Bad name
5- Big Software droughts.
6- Uninventive and unambitious games for the most part (Worst console 3D Mario, no new Zelda or Metroid, 2D DK instead of 3D, Smash Bros. wasn't exactly exclusive, etc. Software wise, the only bright points of the system were MK8, Splatoon and Mario Maker).

You cant blame it all on Name, Concept, Marketing.. ect.

You have to mention the Price to Performance ratio, its end Price, and how expensive the Gamepad was (part in keeping the consoles price so high).

The gamepad is most likely the worse offender.

Nintendo did pretty much everything wrong which they could have.

They trIed to reinvent the wheel again after the success of the Wii and DS, but the market no longer wanted revolution, but familiarity.

Their marketing was horrible and convinced people that the Wii u was a tablet add-on to the Wii instead of a new system, so people never even knew their was a sequel to the Wii to buy.

The gamepad drove up the price of the system beyond what it should have been worth, pushing even more sales away.

Because of the cost of the gamepad, the system was forced to be weaker than it had any right to be in 2012, making 3rd party ports insanely difficult from both 7th and 8th gen consoles.

Instead of trying to reinvent how we play, Nintendo should have released a 'Super Wii' or a 'Wii 2' that was clearly a sequel to the original Wii, improved the motion controls instead of adding a new gimmick that confused people, and was as powerful as the Switch is when docked. That would have sold at least several times what the Wii U sold and gotten many more 3rd party games.

They chip they chose was a bad idea. Choosing a newer PowerPC chip would have been just fine.

Most of the answers have been mentioned a bunch already.

One thing that always bothered me - the gamepad is ugly. It looks like it was produced in 1995. It is way too big for a tablet-like controller launched in 2012. It is also a bit uncomfortable to hold for extended play sessions, as it is heavy, and forces you to turn your wrists more than most modern controllers.

I am happy enough with my Wii U though. I think MK8 is one of the 20 best video games ever made. Xeno was great too, as were some others. I'm glad I bought the system.

Because the casual market collapsed due to smartphones and tablets and the Wii brand had become a toxic liability as a brand that was out of style for one.

A "Wii" system with the "Wii audience" gone to their far more fashionable/trendy iPads and iPhones was always going to crash and burn. If Switch was called "Wii 3" it would be having brand problems also. 

The GameCube did not have Wii Fit or Wii Party U or Nintendo Land or NSMBU ... all central tenants of the Wii success or borne out of the Wii philosophy.

People got tired of mini-game compilations and 2D platformers and guess which two genres Nintendo bet the entire farm on because they did well on Wii for the Wii U's first year.

Terrible marketing aimed at kids and casuals primarily.

Awful controller that looked like a toy.

Bad post-launch lull.

No "epic" Nintendo IP early on in the life cycle.

Take your pick. They're all here.

I think the WiiU is amazing. And its games are amazing. And its totally gamers fault. They've eaten sony's shit and still waiting for ff7remake, shenmue 3 and deep down.

People were burnt out of Nintendo's casual fads by 2011/12. 

It's not a coincidence all of Brain Training, Nintendogs + cats, Wii Fit U, Wii Sports Club, Wii Party U, Mario & Sonic Olympics, ALL underperformed massively compared to their predacessors and Nintendo Land never took off (ironically Nintendo Land is probably the best quality mini-game compilation Nintendo has ever made).

People got tired of that shit. Even happened to Microsoft ... see: Kinect Sports on XBox One .... flopped even though they made Kinect mandatory for XBox One. 

Today Just Dance can't sell for shit. It's the 2nd worst selling game on Switch. The common pattern here seems to be that these types of franchises if they do break out have a shelf life of about 5 years ... after which they crash and burn into something no one wants to touch with a 50 foot pole. 

IMO bad name and poor marketing

The useless, heavy, oversized and uncomfortable gamepad is the main reason. Such a waste of resources. The exact same hardware with just a Wiimote, sold as a Wii-HD would have been a mega success.