That and their desire to explore other platforms as evident in the Halo DS prototype.
Do it ! My DS is in the storage now. I will revive it if Bungie release a DS game
There is no reason why Bungie can't go multi-platform.
It's probably more-so to do with PS3 than the Nintendo platforms seeing as how Rare is already developing a Viva Pinata game for the DS.
Q: Would MS be the publisher on other platforms, or do the Bungie folks have to look for one for that non-MS platform