IIIIITHE1IIIII on 07 March 2013
Super Mario Galaxy 2 was made because they couldn't fit all their ideas into one game. Other than that, Nintendo usually want to wait with their sequels until they can substantially improve the series. New hardware always makes such improvements easier.
(Probably same reason there was no F-Zero for Wii: Online with more than just 12 players would be too laggy while the graphics would pretty much be equal to that of the Gamecube. Additionally, gameplay is way too fast for Wii remotes to be effective controllers. Not exactly a prime opportunity to increase the series' popularity.)
