The most interesting part is, that Skyward Sword isn't the first Zelda game in this bad sales situation. Majora's Mask had the very same problem, it only sold roughly 3 Million and it was released at the end of the N64's lifespan and it required an expansion pack, just as SS now requires the Wii-Motion Plus. Yet is nowadays widely considered to be a fan favourite among the Zelda-fanbase.