He's probably realized that he's not going to be able to win any power for himself by fighting the US forces. I would bet you that, if and when the US does completely leave Iraq, he'll be inciting violence and trying to make a power grab the next day.
Young Jedi Moqtada al-Sadr is not on the best of terms within his own branch of Islam. Perhaps a CONSIDERABLE amount of weight came down on him from within the Shi'a hierarchy. Once he surrenders his rather truculent attitude, he may face some serious retaliation from the more senior mullahs. They do not need anybody rocking the boat just now.