It has nothing whatsoever to do with security, as the DCA flight range limitation has been imposed for decades, with only some recent relaxation.

It is a vestige of the Civil Aeronautics Board days, and analogous to the more severe restrictions applied to DAL when DFW opened.

I suspect the DCA restrictions were imposed to force some (transcon) business to IAD. Now that IAD is more self-sustaining, there is less need to support it though such artificial devices, though it will help when the Metro ever goes as far as IAD.