I think you delved a little bit into it there. If the US has been consistent about one thing throughout the years, it has been its lack of support for - and even opposition toward - leftist revolutionary movements.
Another factor, especially in Iraq, could be the politics of supporting the independence-minded Kurds while also pleasing the Iraqi government and the political elites that dominate it.
I'm pretty ignorant about the Kurds, but from what I've read, I like them more than other rebel groups in Syria and Iraq.
Edit: Also, like Quatrarius said, politics with Turkey and Iran.