Because they will have real power, and they will be willing to clamp down and kill Iraqis in order to keep it.
US forces are limited in what we allow ourselves to do.
But the terrorists in Iraq are blowing up these Shi'ites (and Kurds') families right now, and have been doing it for a long time.
Right now, the Shi'ites do not have real power under the interim government, and the leaders are not in the position to command troops, raise conscription, or order attacks on towns.
Note that the militia failures, right now, are largely in the Sunni areas, because the interim government and the Americans have been careful about trying to use locals to patrol locals.
But a Shi'ite, or Shi'ite/Kurdish coalition government determined to assert itself, will not hesitate to take Shi'ite and Kurdish militias, which do function well, out of Shi'ite and Kurdish areas and place them right in Sunni areas, and they won't have the same sort of careful compunctions we do about crushing opposition. We are careful, for a lot of reasons. For one thing, we don't go in with a generational hatred of the people we are trying to liberate. But they will be going into areas of their former conquerors intending to conquer them.
The interim government cannot credibly resort to conscription, but a Shi'ite clerical government can call conscription, and there are 9 million able bodied Shiite and Kurdish men available for service.
130,000 American troops COULD stop the insurgency, IF we were willing to do what the Shi'ites and Kurds will be perfectly willing to do when they get command of the place. But we aren't. For us, it is a professional duty. But for them, it will be a labor of revenge and solidifying their permanent power over the country.
Even in our attack on Fallujah, we really didn't level the place. The Shi'ites will. And they, along with the Kurds, outnumber the Sunnis 8:1.
Few Iraqis today are willing to join an interim army.
But when the Shi'ites run the country, as a Shi'ite Republic (with proclamations of tolerance, of course, yadda yadda yadda), they will be willing to turn out in force to enforce their will on the Sunnis. Count on it.
LOL. You are engaging in fantasy. What you describe will not even vaguely resemble the outcome if we leave Iraq anytime soon.
If we abandon Iraq in such a state the Syrians and the Saudi will arm the Sunni to the teeth as fast as the trucks can get to them; the last thing they want is an Iranian satellite on their borders. The Kurds will have nothing to do with a Shia war against the Sunni; they will withdraw into their own territories, and if anything hope that the rest kill one another off indefinitely. Baghdad will become Beirut circa 1987 faster than you blink an eye. The Shia will not have even close to the capability of mounting the kind of operation we did two months ago in Fallujah, and they'll need to do that a dozen times over to pacify the Sunni by force of arms. Iraq will dissolve into chaos and might I also add that Iran will no longer be restrained from interfering at will; if Sistani stands in their way he will be assassinated in short order.
My scenario will be very close to actuality. Count on it.