It's always seemed to me on the UA LGA/EWR/JFK-IAD flights that there's not much O/D traffic. Most people have connected through IAD on their way to NYC (or v.v.). This is just observation-based, so may or may not be true (although when they read out the list of connecting flights on the NYC-IAD, it's always very long for a 50-seat plane).

Also, particularly when they're using CR2's, the only possible reason to use UA instead of US/DL is that you live very close to IAD. I fly NYC-WAS often, and will fly UA when they have the CR7/E170 on the route, but will fly US otherwise.