Syria (and specifically the Assad regime) is friendly with Moscow, and provides a friendly port on the Mediterranean. The US hasn't taken out Assad for human rights violations because they don't want a war with Russia. But ISIS is also clearly a threat to the US, so we can't support them either. So we basically tried to invent a third group of non-religious radicals, but without the fervor that a radical ideology provides, many of the individuals that make up this faction give up and flee to Europe after a few months of fighting.

Basically the US would like to see Assad deposed like Gadaffi or Saddam, but Russia is propping him up, and we dislike the other option more than we dislike Assad. The only other realistic option is the Kurds, but an independent Kurdistan would throw the region into chaos (and likely drive Turkey towards Russia, because the Turkish government views the Kurds as being terrorists worse than ISIS).