Congressmen from areas that can serve DCA tend to pick DCA due to proximity/convenience. Noted elsewhere, it's a bit easier to navigate internally (smaller, newer terminal building vs IAD), though I don't know how much that impacts Congressmen - I'm guessing they at minimum are using first class/preferred facilities and not queuing up with us plebes for security.
Periodically, Congress pushes to have DCA take more traffic total and from a larger portion of the US. Currently, DCA is limited to destinations 1250 miles away, though powerful Congressmen have obtained permanent waivers for this over the years for specific flights so they get an easy flight to their own home region.
