The Zelda franchise may not sell as well as Mario games do, but that's partly because the games are directed at a more hardcore audience. Where Mario is accessible and enjoyable to all ages, Zelda games are more intense (and arguably more violent) and require a different kind of attention. With the Legend of Zelda: A Link Between Worlds, Nintendo is trying to attract a more well-rounded audience that skews to a younger crowd on the 3DS. In doing so, they have made a game that appeals to those familiar with the franchise and those just stepping in for the first time.
