The Nintendo Wii has always been about getting more of the family involved in gaming. Rather then seeing it as something they can't join in on, Wii has allowed Mom, Suzy, Grandma, Dad, etc to quickly learn how to play and play along. The Wii U directly goes against this message, almost to the point of being like a "F U casual audience". Let's think about the initial promo video they showed for it.
Wii U Promo Intro:
Dad comes in the room, says "hey, I'd like to watch some baseball". Son says "OK". The TV is switched over and he continues on playing on the controller. By himself.
Imagine the same promo intro for the Wii:
Dad comes in the room, says "hey, what are you playing?" Son says: "It's Super Mario Bros Mii - come play with me". Dad grabs a wiimote and joins in the game. Everyone is playing together!
It's clear right from the start that they're done trying to get everyone playing together. I think Nintendo has realized that the initial casual audience that they went after with the Wii just doesn't buy anymore software or hardware. Now Nintendo wants those "core" gamers back to make up for them losing them with the Wii system. They're essentially saying with this system, especially since you can only use ONE Wii U controller, that it's not about "we" anymore, but "you, the core" - but they hope that they can retain the casual audience.