The DEN and SFO nonstops xDCA are poorly timed. Nonetheless, it can be a lot easier to fly TCON x DCA changing at IAH or ORD than to schlep to IAD and deal with getting from curbside to a gate located 7 miles or thereabouts away.

That said, while DCA is great for O&D, IAD -- as a hub -- has the connections which people need and that removes the incentive for UA to force change. So long as the runways don't crumble, UA has a fortress at IAD and people will continue to use it.