Population of the District =~710,000
Population of Northern Virgina (some of whom are certainly closer to DCA, but many/most aren't) = ~3,100,000 (and it's growing west of Dulles)

Growth potential at DCA = ~0
Growth potential at IAD = ??? (but a lot, witness the thread on additional UA banks at IAD)

So, if you like DCA, good for you. But not for us or for many, many others.