Because Mario Games usually sell at least double the amount of Zelda games.
Your argument is partially right mrstickball, also the PS2 proves that a high install base usually leads to lower attach rates, but you can't compare Mario and Zelda. I mean, go to the city and ask strangers if they know Zelda and ask them if they know Mario...
A lot of casual buyers will try out a Mario game now that they have a Wii, maybe also in memories of SMB.
btw, I actually agree more or less with your lifetime sales prediction for Galaxy, but I think your Halo 3 prediction is off. I expect both games to sell in the 8-10 millions range.







