It's a long story and a lot of factors. But basically look into Bungie and Microsofts relationship before, during and after the split. People like to say that the two don't hold hard feelings but then again any reference to Bungie in a Xbox live profile is not allowed and its a bannable offense. If there wasn't bad blood then I wonder why this would be the case.
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This has been explained by Achronos before. It's not for the reasons you imagine.