I think it is more to do with marketing. The Wii was hyped up a lot more than the Wii U. Nintendo got very lazy when trying to build up the system. Calling it the Wii U is a clear enough statement in saying "You have the Wii, now here's the next level of Wii. If you lived in Japan or America you would be able to know more about it." They didn't try to sell people on it like they did with the Wii.
I should also note that Wii U is going to be far less viral than Wii. The reason being that there were lots of Wii parties with various games, but Nintendo hasn't really released a game that has players aside from the system owner getting a chance to use the new controller. If there were 4 Wii U Gamepad's and they were the right price, and backed with the right software to show off multiplayer, then it would be a hit.
I describe myself as a little dose of toxic masculinity.
