So that Iranian troops can take over the country and other holy places for which Iran is struggling since revolution 1979. USA and other forces must quit and handover security to Iraqi forces and make sure that Iran stay away from affairs of there internal matters
If the US troops leave that region then it's going to be the last chance for stability in that region and the Iranians would have achieved what they wanted to do all along. The Iraqis are nowhere ready to take total control of their own security especially with this US -IRAN tensions currently brewing.