Time is most likely a factor. But it's only known to Bungie employees(specifically in the PvP department). Then again, I don't think Combat Rating should be used as a requirement or determining skill.
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Well my worst combat rating is rift where we Pubstomp without capping any rifts so we don't mercy them. It's my worst combat rating and yet I have a 3.5 kd in it. Hell, I have no clue what combat rating is. :/ It's probably a combination of everything.