Most definitely not a security or 9/11 issue. The perimeter restrictions have been around for a lot longer. Congress authorizes the number of "out-of-perimeter" slots and those are then assigned by DOT. Some of the current configuration arises out of an agreement reached to permit the acquisition of AA by US (and then renaming as AA).

DCA routinely accepts 757's and, has serviced 767's and 777's. But, what it can't accommodate is a fully-fueled and also loaded (with pax) aircraft and then fly to Western Europe.