The Wii's success was mostly a fluke caused by MS and Sony raising prices too much, and a couple of gimmicks that were worth some attention by some: motion controls, and wii fit.
That was no fluke; it was the logical extension of the same strategy that made the DS so successful after a rocky start. Nintendo built a system with a unique feature (motion control), made new IPs that leveraged that feature (Wii Sports, Wii Fit), targeted the nongamer crowd by offering a pleasant "Mii" aesthetic and offered classic Nintendo franchises for everyone else (Mario Kart). The end result was wildly successful.
By contrast, the Wii U is bombing because although it also has a unique feature (gamepad), its new IPs are mostly niche titles (Wonderful 101) instead of mainstream ones and the next iterations of Nintendo franchises are either also niche (Pikmon) or late (Wii Sports, Mario Kart).