It's because the whole system is a mess. Nintendo almost went out of the home console business when they focused on the "core gamers" with the Gamecube. With the Wii U they tried to get both the huge mass market they got with the Wii and get some of that upmarket back that Sony and Microsoft dominate currently. But the Wii U is not mass market compatible: Nintendo's biggest insight with the Wii was that controllers were too complex for most people so they designed the Wii remote. For the Wii U they designed an abnormally big and clunky piece of controller that's even scary looking to myself and I've been a gamer for years. And, of course, they decided to abandon making disruptive products and wanted to focus on the upmarket instead which was a terrible idea.
At the same time the Wii U is also not attracting any core gamers: Nintendo promised ports of 3rd party games and there are none of them. EA is turning their back on the Wii U because they had a fallout with Nintendo about their online service. Most western developers don't want to fully support the system and the japanese devs are focused on the 3DS and PSP.
Basically instead of focusing on either the mass market or the core market Nintendo tried to succeed in getting both - but failed to get either.
