I always thought that rooting for one's "home team" was part of the essence of being a sports fan. I mean, if you are from Dallas and just moved to D.C. for work or something, then rooting for the Cowboys makes all the sense in the world. But if you grew up in D.C. and root for the Cowboys instead, chances are you are just someone "rooting for a winner" beginning back when the Cowboys were good. I mean, why the Cowboys. Why not the Rams or the Lions? What are you basing your decision on, if not geographic proximity? "I always liked how they played" = "They always won, so I started rooting for them so I can talk smack and look better than others..."
A pretty good theory I've heard about the native DC people who like Dallas...The Redskins were the last NFL team to integrate when they traded for Bobby Mitchell in 1962. Redskins owner
George Preston Marshall was generally considered to be a racist. Obviously, that wouldn't exactly sit well with the large African American community in Washington, DC. So, through the 60s, which was obviously a big time for race issues, many people started rooting for the Cowboys. They started their rise in the 60s and became the Redskins biggest rival. A large group of people gravitated to the Cowboys during that time and then likely raised their kids as Cowboys fans while still living in the DC area.
Based on personal observation, I think this theory works. I work in PG County, a predominantly AA county, and see tons of Cowboys fans. Tons of cars with Cowboys flags and stickers. I work with many Cowboys fans. The majority of them are AA. I live in NoVa. Most of the Cowboys fans I know in VA are white and moved to the area from Texas or some other area of the South.