The tribal solution in Najaf - why now?

It is reported that agreement has been reached to terminate the US offensive in Najaf and to hand over security to a force drawn from local tribal militias. Moqtada al-Sadr's army will, supposedly, evacuate the city and cease operations, though not necessarily disband. All very good, but why didn't it all happen much earlier? Back on the 5th of May, I blogged that the US Army and Shia powerbrokers had come around to something like that, quoting this from Informed Comment. However, it didn't happen, allegedly due pressure from the CPA, and the battering began. Question: if this option had been taken then, wouldn't it have wound up the fight for Najaf just as much as it has now - without shooting up the Imam Ali mosque or driving tanks all over the cemetery?