Nintendo's HD console has had a rough first year, and by the looks of it, its situation likely won't improve all that dramatically in the coming years. According to DFC Intelligence, Wii U "sales [are] anticipated to be only about a quarter of the Wii."
As for why this is the case, DFC explains that the console "is expected to be mainly a system for Nintendo first-party software," resulting in a limited library, and thus, a smaller install base. Wii was an anomaly, and since the motion gimmick has lost its allure and the tablet controller isn't wowing consumers, Wii U will likely continue to sell poorly.
Of course, this doesn't necessarily mean doom and gloom for Nintendo fans, as GameCube wasn't a very successful system for the Big N either, depite being home to some of the greatest games Nintendo has ever created. Here's to hoping the same proves true with Wii U.