Quote from NPR:
Right. You know, at one point, it was thought that a contingent of several thousand American troops was going to remain in Iraq into next year, sort of, beyond the agreed upon deadline, as a force of trainers to train Iraqi security forces, to help get the security back up to speed. But, you know, there was a - the talks between the Americans and the Iraqis pretty much broke down sometime this fall over the question of immunity. American soldiers, if they wanted to stay in Iraq, wanted to know that they would be immune from prosecution under Iraqi law. And the Iraqis, frankly, after eight years of war and more than 100,000 people dead, just weren't willing to grant that.
Most of them inflicted by Sunni Iraqis and Iranian supported Shi'a Iraqis on each other.