I'm going to go a different route to most people and tell you that it costs about 5 times the amount of money to attract a new consumer than it does to retain a current one.
Nintendo need to ensure that people who have bought a Wii see that it provides value for it's entire lifetime. The more customer they make happy, the more that will return to them. Brand loyalty is a powerful thing.
They basically just add value to the system.. If people hear all these good things about Mario Galaxy and then see that Galaxy 2 is coming out, even better than the original, then they might decide that now is the right time to buy a Wii. If people see that Wii Fit has been replaced by Wii Fit Plus and it has more activities and more features, then the people who were interested may actually decide to buy it because of the initial hype and then following word of mouth.
It's also the fact that Nintendo are about the games and they let their teams be creative and if they feel there is room for a sequel then they won't stop them.
Last thing I wanna say is that Twilight Princess was a title made in Nintendo's older value framework. This Zelda will be designed in the newer value framework which seems to be accessibility, fun and intuitive and engaging controls before everything else. They have always focused on the experience but this one will focus more on being easy to pick up and play but hard to master, much like Mario Kart Wii.
