Density, but also just time. NYC has been a huge city for centuries, and a lot of those cheap mom and pop shops have been there for ages. A good number of them even own their stores free and clear, which helps keep the monthly costs low.

DC has not been a bustling city for nearly as long. We didn't even break the top 10 cities/metro areas (by population) until like the 1950's. So we've had less time to establish a good working class food culture.

Also, the food culture that was established was mostly destroyed during the MLK riots in the 1960s. H st and U st used to be bustling African American commercial hubs that could have been the basis for great mom and pop shops if the strips hadn't been burned down and then neglected.

Then, like 20 years later you have the crack epidemic, "murder capital of the US" and all of the other issues that lead to 90s-00s suburban flight. Then in the 10s and 20s you have gentrification and now COVID.

There's unfortunately a lot of events that have kneecapped DC's food scene before it was really able to get started.