I have a theory that the reason we aren't getting more Mario* is the fact that Galaxy is not even a year old.
...But was delayed along with Brawl and Metroid Prime 3 to see release at a later date. Now, obviously, that probably helped improve the game, but that doesn't mean it had any bearing on the development of games even further in the future.
As for me, I'm not really expecting more Mario or Zelda soon. It took about a year to make Phantom Hourglass and... yeah, we all know what I'm going to say here.