Because the parts of DC that you can safely, raise a family in are either:
1) Really expensive
2) Essentially suburban anyway. Most of NW is suburban.
3) both 1 & 2
Another cause is that until 10 years ago most of the city was considered to be too unsafe. Gentrification has gone into overdrive and has greatly expanded the area middle class and UMC people will live. But even then a lot of that area is still pretty crazy in terms of cultural stuff. And it isn't as safe as people claim. There has been a recently uptick in violence. People getting murdered on the metro. A home invasion that led to a family getting massacred. A guy got shot in a luxury high rise in NOMA.
Safety is relative. Consider that by living in a walkable area avoiding driving, you may be reducing your risk of being one of the ~40 automobile-related deaths a year in the DC region.
You don't have to just worry about murder. You have to worry about rapes, muggings, and property crime. Nobody wants to live in a place where you have to "watch out" or stay inside at night.
Luckily DC has some pretty urban suburbs like Arlington and Alexandia. They are more urban than most of the district. I don't have to worry about my car getting broken into and I can take the blueline to work.
I'd prefer to live in Georgetown or DuPont but that stuff is insanely priced.
What NOVA and Maryland need are more and better commuter rails to DC. The VRE is awful.
1) Really expensive
2) Essentially suburban anyway. Most of NW is suburban.
3) both 1 & 2
Another cause is that until 10 years ago most of the city was considered to be too unsafe. Gentrification has gone into overdrive and has greatly expanded the area middle class and UMC people will live. But even then a lot of that area is still pretty crazy in terms of cultural stuff. And it isn't as safe as people claim. There has been a recently uptick in violence. People getting murdered on the metro. A home invasion that led to a family getting massacred. A guy got shot in a luxury high rise in NOMA.