al-Qaida is also present in Syria in the form of Hayat Tahrir al-Sham, previously Nusra Front. They have Afghan veterans in their ranks, but do not have the same level of sofistication and standardization as the Islamic State.

It's not obvious exactly why the IS is different, but I doubt it has anything to do with American influence. Other rebel groups in the Syrian civil war has had direct American training but they were highly undisciplined and ineffective.