Since the U.S does not have clear and detailed policy towards the conflicts in the Middle East, and since the U.S policy is currently anchored in the wait-and-see foreign policy, Washington is more willing to delegate the task of fighting the Islamic State or resolving the crisis in Syria and Yemen, to Tehran and Moscow or other nations.
Times are hard in today's Russia. At some point, Russians will expect their powerful leader to deliver better times at home. To delay that day of judgment a little longer, Putin needs victories -- particularly those that burnish Russia's image as a world power. Once again, he's getting them. For now, he's on a roll -- and an international force to be reckoned with.
Assad is not only an individual who can be replaced by someone else, but he is an indispensable part of the Syrian state; he embodies the domination of Alawite in the political establishment. The removal of Assad from power will be a strong blow to the Syrian government, and a moral boost to the oppositional and rebel groups.