Uh... Microsoft did not make Halo.
Bungie did, which Microsoft acquired with its pocketchange
and yanked the PS2 release, the Macintosh release and the PC release...
Why? You do the math.
Microsoft's games are insipid, lacking in creativity and the good ones... are not even theirs.
Their tactics for the gaming market? Not make better games, but buy the small, talented, creative think-tanks that produce them. If you don't understand this, go to Japan and see how poorly the XBOX is doing, a country whose gaming devs produce games because of the loyalty to their fans, not empty pockets.