The Will of God, obviously - mainly as realized in the form of geography and the evolution of the human species. Some have pointed to certain critical inventions, such as the stirrup and the turnplough, or "guns, germs, and steel," or the rifle, or modern mass governance, and so on, but there are chicken-and-egg problems those, while a geographical determinist can make a case that the general course of development was either set or radically narrowed by at latest around 10,000 years ago.

There is no fortress Saudi Arabia or fortress Egypt or hermetically sealed "Arab World" or "Middle East." Currently, there is a vast disproportion in wealth and in human or social capital by numerous objective measures, while nature fills that which is empty and empties that which is full. Simply on that level, the West (objectively) is drawn to "fill" the Middle East as inexorably as solid objects in the air are pulled down by gravity. What do you imagine is supposed to occur instead - even before we consider the long history, or virtually the entirety of what we call history, of interpenetration and critical conflict between the regions and their cultures.

Put differently, the Middle East rests on the "crossroads" of civilization between the two great economic systems of the world: The Asian, land-based system whose backbone was and is the Silk Road, and the maritime system, formerly the Mediterranean, now the global oceanic system (encompassing the Western Hemisphere, which prior to the Age of Discovery was mostly segregated). You can fantasize about everyone leaving everyone else alone, but to make it real you'd have to lower the average temperature of the surface of the Earth to freezing.

From this perspective, the continually interrupted "Pivot to Asia" is just an attempt to return to the same conflict by a different angle of advance. The problem is that converting the Middle East from central front to rear area will be strategically impractical - and dangerous - as long as that rear area remains unsecured.