because American special forces have been fomenting mayhem ever since the newly installed government in Iraq proved no more keen to have a long-term U.S. military presence and give us basing-rights as Saddam Hussein was.  
Since the bases on the eastern side of the Indian Ocean basin were abandoned to the Vietnamese, the only practical alternative was on the west.  Saudi Arabia didn't work out.  Yemen and Kuwait are, for some reason, not large enough or perhaps too distant from the area of interest (China and Russia).  The containment of a "rising China" and a "re-emerging Russia" has been the objective all along.
For a while, it seemed that a space-based missile defense would be practicable, but that idea has now been debunked.  Besides, China has demonstrated that it can shoot down satellites whenever it wants.  So, the only alternative, if you're going to defend against missiles that have already taken off, is to have boost phase interceptors (hit them before they climb into the stratosphere) and that means taking off less than 50 seconds after lift-off. Which means they have to be pretty close and can't be undersea.  So, Iraq seems ideal and McCain seems to remain convinced that eventually the mayhem will convince the Iraqis to cry uncle and let us stay there.
Never mind that it's unlikely that Russia and China will agree to our unfettered presence on the southern edge of Asia.