Sadly it was actually supposed to be a 360 exclusive and later be upgraded to Xbox one
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Bungie said them selves, after seeing the amount of money Gears of War raked in and the money the devs got from that, compared to their pockets from Halo, they new they had to make a change. MS was -blam!- them. From the interview I watched I didn't see them say it was originally for 360, so not saying you are wrong just that they had to leave blood sucking MS to make some money for their effort.
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There obviously was a good reason for them to leave, and I'm not saying that they shouldn't have... I'm just saying it was originally in production for Xbox 360 back before halo reach(at least the idea).