Don't underastimate the power of fanatics and lunatics. It can do wonders if they are properly equipped. Not saying anything about Muslims though, don't get me wrong, it's just that those extremist have a strong motive in jihad and Kur'an, which is probably the main reason they are winning.
Motivation alone doesn't explain their gains. The Kurdish Peshmerga fighters are even more motivated, when compared to the ISIS terrorists. Actually, it is the technological advantage which is aiding the ISIS. Right now, they are having some 2,300 US-made Humvees and tens of thousands of heavy machine guns with them, while the Kurds are armed only with light machine guns.