The Saudis are certainly part of the picture. They have a genius for speaking out of both sides of their mouths: posing as pro-Western "moderates" when they need something from the US, while assuring their own militant constituents that they're really on the side of militant by implementing sharia law at home and supporting jihadi fighters throughout the world.
I doubt that their influence is the whole story, but they're a big part of the picture. It's no accident that you had both Obama (literally) bowing to them and George W. Bush (literally) holding their hands.
Simply stated, the Saudis keep their friends close and their enemies even closer.