What are the options? 
1) Do nothing: Assad's regime kills hundreds of thousands of innocent people. The rebels are backed by countries like Turkey and Saudi Arabia, and can keep the war going for years. 

2) Get in there, fighter jets ablaze, and enforce a no fly-zone over the country to block the Syrian air force. Russia is unlikely accept this, so it would probably mean having to engage the RuAF directly, which could have massive repercussions beyond Syria. Furthermore, what then? Do the rebels win? The leading rebel groups are Islamist theocrats, they'll wipe out or subjugate any minorities who don't subscribe to their brand of lunacy. To say nothing of all the infighting between these groups causing further instability, that's just replacing one tragedy for another. 

3) Boots on the ground? Do we want to buy into yet another fantasy that we can set up a liberal democracy in the Middle East if we throw enough money at it?

This isn't an exhaustive list, but all roads lead to Rome.