Well, Candice, it isn't those proverbial "root causes" of poverty, unemployment and the like, the reasons "progressives" like Obama always point to as the impetus.
What's driving them? Isn't it obvious? It's the siren call of jihad, which is particularly potent for certain idealistic youths who may be looking to channel their youthful idealism into a "worthy" cause. And, clearly, working for "social justice" and/or to put an end to global "climate change," causes which are amorphous and non-specific, don't captivate these youngsters in the way that running off to fight for the Caliphate does.
Simple as that, really.