DCA is just across the Potomac River from DC and thus large amounts of restricted air space. This makes take offs and landings at DCA challenging since all this restricted air space has to be avoided. DCA also has not space to expand to try to mitigate the risks. Congress has interfered over the years by attaching riders to legislation requiring the number of flights allowed in and out of DCA to be increased over the objection of the Metropolitan Washington Airport Authority (MWAA) who runs both DCA and IAD (Dulles). This despite decades of warnings that the number of flights posses a significant safety risk.
Regional airport authorities which run multiple airports such as MWAA generally spread the cost of improvements out across the multiple airports by increasing gate fees at all the airports they control to cover the cost of improvements at any one airport. Congress has forbidden MWAA to do so which has limited their ability to expand and improve IAD to lure airlines to shift domestic flights from DCA to IAD. IAD remains primarily an international airport with domestic flights in and out supporting that role. This is largely due to the higher gate fees at IAD.
Regional airport authorities which run multiple airports such as MWAA generally spread the cost of improvements out across the multiple airports by increasing gate fees at all the airports they control to cover the cost of improvements at any one airport. Congress has forbidden MWAA to do so which has limited their ability to expand and improve IAD to lure airlines to shift domestic flights from DCA to IAD. IAD remains primarily an international airport with domestic flights in and out supporting that role. This is largely due to the higher gate fees at IAD.