The reason? It's an ignorant ideological approach, which doesn't deal with reality and in the process abandons true statecraft. We have created a state of war in Iraq and international crisis and instability with our fraudulent neo-Rhodesian invasion of Iraq. It was the dumbest thing we could have done. Iran isn't becoming the regional power, it is the regional power. The Saudi's aren't a military power worth speaking about, they are an endangered regime.
You can't isolate Moqtada from the so called Iraqi government, he is one of two pillars of its existence. He holds the balance of power. Whether he is with or against the Bahdr militia determines whether a Shia dominated Iraq will exist.
We have no choice given Iran's pre-eminence in the region but to withdraw and back the Sunni insurgency. Without our presence to complicate the civil war, the warring factions will be forced to negotiate with each other, not us.
The object isn't a Sunni dominated or Shia dominated government- it is to force the parties to reach a political accomodation. With us out of the picture, a temporary cease fire would be possible. The current government is irrelevant. I don't believe a cease fire would last but it would give warring factions an opportunity to stake out their positions. Conditions not met will result in further attrition. Attrition will force the parties to come to the table again. After repeated cycles of negotiation and violence a new government will be formed and the Bremer colonial constitution written by Americans to advance their globalist corporate agenda will be thrown on the ash heap of lost causes where it belongs.