It was a perfect timing. MS was bringing in developers, and Bungie was the perfect target because it needed help, and because it was the coolest IP in development at the time. The trailer they showed at E3 2000 was the bee's knees, and ended up having little to do with the end game's actual plot, but it sure sold MS on the idea that this was killer IP. It was actually a compromise to bring Halo to the console because it was an FPS, which was not really a console thing, outside Goldeneye. But then, MS had signed Rare, too.

Isn't it obvious MS was buying up things it specifically thought were cool? The original Xbox lineup was a who's who of bad ass computer game devs like Molyneux and the Rare folks. And, clearly, all of the studios were able to negotiate decent contracts because they all were free of MS in the end, unlike other studios that have had their publishers run them out of business rather than see them keep their IP. In the end, bungie was glad to be rid of Halo, actually.