Now you tell me first:
What did the WiiU do to deserve to become a new generation?
Sure, it might have better specs than PS3 / Xbox360 (though I don't now exactly, because I too think that a new generation can no longer be based on specs); but what does it add? A screen on the controller? Mobile gaming had that figured out years ago (all right, minor sarcasm there, I apologise).
But yeah. That is basically it. What else did the WiiU add that we were unable to do before? It rises above the Wii in terms of specs, that's for sure, but it's not regarded as something spectacular anywhere. Without any real upgrade there is little left to fight for.
Now here's a twist for you:
It does not matter. Nintendo is simply no longer taking part in this "generation arms race". The same thing Apple did with the iPhone. It's good, it's quirky, there's demand and all in all, it's just something different.
There's also a catch: I don't think Nintendo is having enough third party support to guarantee a long lifetime for their consoles, which in turn cause the remaining interest to decline even further.