As much as I would like to take the simple answer that the only differences are guns are allowed in VA and not in DC and MD, it isn't the only major difference, and the other factors certainly play a role. What might change some is if law abiding citizens would/could arm themselves for protection. However, given the demographics of these areas I find it unlikely that a great number of people would arm themselves for protection.
I lived in PG County Maryland in an apartment that backed to District Heights. I have been there and done that when it comes to a high crime neighborhood. I'll bet Janq and some others know where I was and how nice a neighborhood it was, and can probably guess how often I had "issues" because I am white.