Why was the Wii U a Failure
The Wii U tried to repeat something unrepeatable, therefore failure was inevitable
The original Wii
The Wii's casual nature forced it right through all the charts at a never before seen pace, it brought a whole new audience of casual gamers, and even new people who had never played video games
Trouble in paradise
Later on in the Wii's life these casual gamers, the same ones that made the Wii as successful as it was, either stopped playing video games, or moved on to smartphone and tablet gaming.
Wii U's European Sales
Nintendo had predicted that they would sell at least five million units by March of the consoles release, however over half a year later the console could not even manage to sell 4 million. Now compare that to the sales of the Xbox One and the PS4 which sold at least 1 million units...int the first 24 hours
Wii U Software Sales
The Wii U's poor sales are even represented by it's software, third party support is drying up, and first party classics are struggling.
Many game companies such as Ubisoft are even pulling away from Wii U out of fear that the game won't sell or it will give them a bad name
"They wanted a console that wouldn't make much noise, so'mum wouldn't mind having having it in the living room" - Nintendo executive. Sure in the 90s it would have been fine to assume that Adults or even older teenagers don't play video games, however nowadays that mean that you are missing out on a huge group of potential buyers, with a lot more buying power than a mere child.
Wii U's Other Setbacks
Remember when all of your information was tied to one console? Well you can now relive that with the Wii U.
Another setback for this console is the graphical power. While other companies are dropping millions to switch from SD to HD Nintendo is just settling at what they have now, This causes a whole other group of gamers to be lost over to PS4 and Xbox One.
Wii U vs other Console's Graphics
Why the Wii U failed?
The Wii U failed due to terrible marketing, hardware that was not up to par, and awful connectivity.
Do not fear, Nintendo will be fine, They can bounce back form this, the 3DS is not going to stop flying off of store shelves anytime soon, and with the release of Super Smash Bros. the Wii U will sell even more units.