Besides the poor marketing and the uncreative name, the easiest reason why the Wii U failed as a console, has to be the Gamepad itself. There are a few issues the item has, and I will discuss them.
If someone were to go outside and turn on the Gamepad alone, it will not be able to connect to the console, leaving you unable to play the game.
During gameplay, the Gamepad will by chance randomly lose connection with the Wii U and turn off, making you lose control of the game you're playing. This is especially a problem if you are in the middle of a challenging level or boss fight, or even worse, in the middle of a Turf War in Splatoon, which is unpausable.
Turning on the Wii U itself is fine, but it's so annoying that you have to also turn on the Gamepad to really activate the console. Why does the Gamepad even have a Power button?
If the Gamepad is low on power, you have to keep it charged, or it will shut off and cause your game to pause. The Gamepad's battery life isn't very long either.
You are required to have the Gamepad to properly play the Wii U. If you lose the gamepad or if it breaks, you have to by a new Wii U console.
Here is how the Wii U console should have been designed or produced:
Instead of a large and bulky Gamepad, the Wii U comes with a smooth controller with a cord, just like several previous Nintendo consoles. The back of the console has a pad for scanning the Amiibo figures, instead of needing to do it with the Gamepad screen.
Had the Wii U been created like this, maybe it would have been a more successful console, alongside the Playstation 4 and Xbox One.