Wiped on 08 November 2008
Nintendo don't whore out their franchises, that's why. This has so many advantages - it gives the games massive legs, and reduces the amount of traded in 'old versions', like what happens to every HD game when the next yearly iteration comes out. This also stops people becoming bored of the franchises - Nintendo's IPs have lasted 20+ years for a reason. Finally, it means they have lots of time to polish and perfect the next one.
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