The regional parties involved in the Syrian war are:  Turkey, Saudi, Iran, Russia.  None of them wants democracy in Syria.  None of them wants an independent state in Kurdistan, either.  They have been regional powers for a thousand years.  Their staying power is eternal.

Only the people of Syria, America, and the western democracies want democracy in Syria.

Turkey and Saudi Arabia have made sure that no democratic force could succeed.  They only fund and support Islamic forces.

This means that the western countries have nobody they can support:  There is no public support for a third war in SW Asia, and especially not one that can only result in another undemocratic Islamic dictatorship being created.

It is like the Spanish Civil War:  Once Mussolini, Hitler, and Stalin moved in, there was no "moderate" center for the west to support.  The extremists demanded doctrinal purity: they killed the moderates first.