As with many infamous disasters throughout history, a lack of imagination and expectations is almost certain to have played a key role.
I personally think the success of the strike is as much about the Saudis not expecting such an audacious attack and getting caught with their pants down as much with the brilliance of the execution of the attack.
The Saudis’ procurement choices are also likely to blame, as they have heavily, and often over invested in ‘sexy’ offensive weapons, with little to no regard for key defensive and support systems like SAMs, AWACS, long range early warning radars etc.
I foreseen big ticket purchases of SAMs and early warning radars etc in the immediate future. China’s new AWACS drones might be especially attractive, as they would be ideally suited to detecting low flying drones and missiles, and would be possible to mount continuous, near open-ended operations that would tax manned AWACS fleets to exhaustion.