Why Syria? What is so important about Syria?
The short answer is Israel.
Syria is the gateway, at least the primary gateway into Israel from the north, it is in the South where Hezbollah and other Iranian backed terrorist groups have waged jihad against Israel for decades. The north however is where it is believed by most geopolitical experts that air superiority would have to be achieved if Israel's enemies were to achieve a "final solution" to the Jewish State as has been reiterated by such enemies as Iran for example.
All of the players in the Syrian Civil War all have Israel in mind. Iran and Russia and the Assad Regime are all playing the long game with Syria's border with Israel in mind.