Iraqi and American counter-terror efforts have reduced the number of terrorist attacks there by over 90 percent in the last few years. But the Islamic and Sunni Arab terrorists are still at it, at least a few of them. The terrorists now concentrate their attacks on targets that are likely to be noticed by the media. The terrorists are still trying to trigger a civil war between Sunni and Shia Arabs. But the Shia are still firmly in control (they comprise over 60 percent of the population), while the Sunni Arabs are down to 15 percent, with over a million of them still exiled in Jordan and Syria, afraid to come home. Many of the exiles worked for Saddam, have blood on their hands, and are afraid of retribution if they return. For some of the worst cases, they are probably right. Saddam's murderous secret police and enforcers did not hide their identity as they terrorized Kurds and Shia Arabs, so many are known, by name and on sight, to the family and friends of their victims.
Their identities are also known to American intelligence agencies, along with a lot of other interesting bits of information. For this reason, the Iraqis still want the Americans around. Currently, there is an American combat division surrounding Baghdad, and several other brigades at strategic locations in the country. These 120,000 troops are to be gone in a year, but the Iraqis want to stay in touch with American intelligence agencies, and would like some U.S. Army intel units to continue operating in Iraq.
All this is because, the Iraqis have enough reliable police and soldiers on the ground to keep terrorists on the run. But they need that primo American intel to hunt down the remaining terror groups. The U.S. is glad to provide it, especially since this information prevents Iraq from turning into a police state (the only other known cure for persistent terrorism). There are Iraqi Shia Arab politicians who be fine with a Shia Arab dictatorship in Iraq. Iran would be more comfortable with this, although the surrounding Sunni Moslem states would not be. So in the name of preserving Iraqi democracy, the U.S. keeps its combat troops in, or near, their suburban bases, and keeps the intel tap flowing.