Because Assad is in bed with Iran.
True.
The rebels are no fans of the Saudis or other member states of OPEC either if I read the news correctly.
So unless the members of OPEC are willing to fight the rebels for control of Syria after Assad is gone I don’t see an upside for them in deposing Assad. They will simply have supplanted a dictator that is unwilling to cooperate with them for a chaotic state with fighting factions that is unwilling to cooperate with them.