Like Dulfite said, the branding was a stupid idea. People couldn't differentiate the Wii U from the Wii and therefore confused it with an overly expensive add-on. Also it partly came in the same white colour as the Wii. It was even more confusing when you consider the fact that Wii nunchucks are a valid controller option in some games.
The few million people who actually bought a Wii U indeed love the damn thing, but the general audience has no idea what it does or that it even exists.