I figured it's one of two options: A) Microsoft figured it wouldn't be a good deal because the game wouldn't sell good/wouldn't have a lot of players (and as you can see from everyone quitting and how people say it's not a full game I could see this being a reason) B) Bungie didn't want Microsoft because of how they treated them in the past when Microsoft said Bungie could only make Halo games. That could've soiled their relationship so Bungie wanted Sony instead.