This is pretty much my opinion.
And it's genetic. My daughter has broken up with two guys because she found out they were Orioles fans, and knew she wouldn't be able to take the static.
Bravo!!! (Around 1959 or 1960, there were big arguments between "us" and people who turned their TV antennas to get Baltimore TV so they could root for the Colts. Same with Baltimore Oriole fans. We called them "traitors")
Additionally: DC is not particularly more transient than any other big city. Kids grow up, go to college, and get a job someplace else. My son and daughter-in-law, and two grandsons, live in St Louis...son is a fanatic Redskin fan, and older grandson's first coherent sentence was "Goal Ovie!" Son is a Nats fan -- gave me "The Wrecking Crew of '33" -- but daughter-in-law is a Cardinals fan.
While some of the DC work-force changes with politics, most people are civil service, and the area developed a pair of computer concentrations along I-270 and I-66.
I find it pleasant that many New Yorkers wish me well when they see the curly-W hat. Maybe not Met fans, but there aren't so many Met fans. Yankee fans often say things like, "I'm really impressed with the Nats...fine team...they've been built the right way".