How well ISIL fights is actually a clear metric of the inherent legitimacy they possess. We would do well to ponder on that thought.
That uncomfortable thought is certainly part of the reason. I would add that a unifying ideology is critical to bring together civilians (in this case from around the world) and turn them into a disciplined fighting force. And of course, a statement of the obvious, they must have excellent trainers. It also appears they're a learning organization.

A lot of folks we train around the world don't believe in their cause (probably most, thus the value of the legitimacy argument), they have weak and dishonest leaders they don't trust, and they're not learning organizations capable of adapting (based on poor leadership). There are a lot of intangibles involved in unit effectiveness that will undermine our tangible capacity to train and equip.