My main take from what Ive read is that the US has not only been reluctant (for years now) to robustly back anti-Assad forces due to fears that'd be backing islamic extremist movements, but that principally they are more concerned with the nuclear deal with Assad's main backer, other than Putin, Iran. Overtly helping the Kurds and angering NATO member Turkey a distant second to this. A messy failed state "worse than Assad" may also be posited as a secondary reason too.
Related thought: interesting that ISIS's social media drew upon them an (ultimately "ineffectual"?) limited air campaign whereas Assad has killed and displaced many more than ISIS had to date...also the interesting de facto military alliance of Shi'ite Iran and US vs Sunni Iraq while Shia sect Assad is left free to slaughter with Shia Iranian $...while at the same time the nuclear talks occur over in a totally political arena featuring the US "against" Iran. Then you got the Saudis (US formal ally) and the Yemen Iran proxy thing.
Very messy..and we havent even gotten to Egypt or Israel!