I have flown this route often when I worked in NYC and lived in DC, and noted many Europeans connecting from/to their flights to/from Europe out of JFK.

Also, UX has an extensive network out of IAD to many secondary cities in the mid-Atlantic areas which EWR does not serve.

IMHO, LGA is more for regional traffics between the DC/NY. JFK has more traffic for connecting flights for *A partners.