There are several reasons for it!
Russia`s opinion is that only people of Syria can deside, who is the president of Syria. This is not for Obama. President of Turkey told it Obama even (it was when Obama wanted to change situation in Turkey).
Russia`s opinion is that only democratic general election can show the name of new Syrian president, and even if it is current Mr. Assad.
Russia`s opinion is that ISIS and other terrorist groups funded by USA are the main obstacle for democratic general election of new Syrian president.
So, new Syrian president can be elected after antiterrorism operation only.
Assad nowadays is the only power, that fights with ISIS and other terrorists. Nothing special! For ISIS Syria is just a one of plenty targets, such as Mecca, Medina, Jerusalem, Europe, Russia...