When Nintendo first unveiled their Wii U console at 2011's E3 conference, people expected Nintendo's new system to be completely seperate from the Wii. Names that were going around forums and social networking sites were odd. People have reported that Nintendo's 8th generation console was going to be called simply "Nintendo" which obviously didn't happen.
There are still a lot of people in the world that don't know what Wii U is. They are used to seeing many different Playstations and Xbox upgrades, but not a Wii upgrade. If Nintendo decided to name the system Wii 2, there would be absolutely no confusion and people would know that it's a completely new system. With the tablet controller, people think it's an add-on for the old 2006 Wii. Nintendo has to put more focus on the console itself than the GamePad to convince consumers that this is an entirely new system that is much more powerful than the Wii.
What if Nintendo ended up naming the system something else? They could've just called it "U" but people wouldn't know that it'd be compatible with Wii accessories. They wanted to keep the Wii brand going as they clearly aren't done with it yet. Since original Wii units are still selling, it'd be awful for Nintendo to pull the plug on their most successful system yet. The Wii U will give the Wii brand another 6 or 7 years, but what about after that? Wii 3? Wii X? Nobody knows for sure.
Nintendo has lost quite a bit of support after announcing the Wii U but only because the original Wii was more of a novelty item than a hardcore gaming device. Once Wii U pulls through and more games are released, we'll see more support for the system. On the other hand, if Nintendo called the Wii U something completely different from the Wii and if they added a bit more RAM and processing power, they'd have an instant success on their hands. Sadly, this isn't the case, but the Wii U is still young and we still haven't seen it's full potential.