Why can't you and 5 families buy a piece of land in Spring Valley and split it into 6 smaller, less expensive pieces and build 6 separate houses? Because that is incompatible with the "neighborhood character." Why can't a non-profit organization buy a property there and build subsidized apartments for low income families? Because apartments would negatively alter the "urban fabric." Apparently there is something inherently wrong with putting anything other than a mansion next to a mansion, because almost all zoning codes prohibit the mixing of uses and building typologies on the majority of the land under their jurisdiction.
American Community Survey 2005-2009 5-year estimates
DC GIS shapefiles (dcgis.dc.gov)