The Wii was able to exploit a perfect storm of marketing. The novel motion controls garnered a lot of media buzz, and it certainly helped that it launched at half the price of the then new PS3 and XBox 360, while including a pack-in game, which made it a more convenient "single purchase" holiday gift. It's graphical shortcomings were excusable, at least for the first three or four years, given the low cost, novelty of motion controls, and the low market penetration of HDTV's at the time of its launch.
Nintendo followed this up with the Wii U, with the Gamepad as the new hook. Unlike the masterful job they did with the Wii, Nintendo failed to effectively convey to customers how the Gamepad works, how it is used in game play, and that it is part of a bundle that also includes an entirely new game console, rather than an add-on for the original Wii. The Gamepad also boosted the manufacturing price. Given Nintendo's insistence that the console itself not sell at a per-unit loss, the resultant retail price negated much of the price advantage the Wii had enjoyed at launch.
Unfortunately, unlike Microsoft, who was able to realize an instant $100 price cut by simply jettisoning the white elephant Kinect, the Gamepad is so tightly integrated that releasing a lower cost Wii U bundle with a Pro Controller in lieu of the Gamepad most likely isn't a viable option. Fortunately, Mario Kart 8 sold 1.2 million copies in its first weekend. The (anecdotal) fact that none of the dozen or so Gamestop's within a 20 mile radius of my house have them in stock also bodes well, I think.