Probably because MS ownes Bungie.
As for MS's influence on Halo, I don't know how true that is. We do know that when they bought Bungie they pretty much did so in order to take Halo, make it a SP game, and build a franchise around it. As such, Bungie had about a year to pump out a game. I've never heard anything about MS pushing Bungie to put the flood in or artificially extend the length, but seriously, even if they did - without the Flood (which I disliked) there wouldn't really be a story, and without the repeating corridors the first game would probably have been an hour long.
Sure, they may have rushed Bungie, but looking at Halo 2, maybe Bungie just doesn't know how to make different rooms.