It’s convenient for everyone. Normal, local people (including myself) tend to strongly prefer DCA when a route is available in/out rather than deal with Dulles or BWI.
A big deal is being made in San Antonio that we now have a direct flight to National. We need to keep up with the Joneses.
Which is why it will not close. It's easier for Congress members, senior staff, etc. to use rather than the drive out to Dulles of BWI. It's convenient for powerful people.