A quick pullout would leave a vacuum that Iran would be all too happy to fill. The Iraqis who are supporting a democratic government would be completely demoralized. It could lead to a very quick unraveling of national cohesion.
U.S. forces must walk a tightrope. They must restrain the violence while letting the Iraqi government forces take the lead, yet on the other hand they don't want to clash with any of the (legitimate) factions.