Sluggo is correct. When IAD was opened in the early 1960s (with the then-futuristic Eero Saarnien architecture) it was thought to be too far from central DC (although the opening of the Dulles Airport Access Road helped ease it a bit). This was before Fairfax and Loudoin counties really became developed. And for years, it was thought to be a white elephant airport because DCA was so close (and overcrowded).

IAD really started to become a major traffic airport after the deregulation of US aviation in the late 1970s. It will continue to be one given the operational restrictions/limited room for expansion at DCA, and the economic activity around the airport area.

I will add that IAD sure could use some refurbishments and renovation...the main terminal building still looks cool but is just overcrowded. And the midfield concourses are narrow, dark and cramped as well, especially when compared with other UA hub concourses at DEN and ORD.