The reason that the Wii U isn't selling as good is the same reason that the Wii sold a crapload. The ones buying the Wii weren't the hardcore gamers that wanted the latest and greatest. They didn't care about next gen graphics, they didn't need octo core GPUs with terabits of RAM. They weren't the guys shouting racial slurs at each other in deathmatches. They were people who never gamed before. They were sorority sisters and grandparents. They were people looking for something a little more than farmville. They were people that just wanted something to have fun with. They wanted Mario Party, not Halo.
The problem now, is that these people don't see any reason to buy another console when the one they have now works just fine. Like I said, they aren't technophiles that need the latest console. Nintendo's market is full of people that won't buy a Wii U until their Wii breaks down, and even then, they'll probably just get another Wii since it's half the price of the new console.