[jcranmer]

As my sibling comment points out, VA sprawled a lot further than MD did. The US-15/Quantico line in the VA is really quite close to the boundary between suburban sprawl and true rural. Cross the Potomac, and you cross from sprawl on the VA side to rural lands on the MD side: the western and northern reaches of Montgomery County are definitely rural, similarly for the southern reaches of Prince George's County.

An additional factor to consider in the DC area is that the DC central business district is relatively weak compared to other major jobs centers: Arlington, VA (just across the river) has hefty job concentration, as does the Dulles-Tysons corridor; on the MD side, there's an additional jobs concentration on Rockville-Bethesda.

The final factor is of course the Baltimore-Washington divide. As you head northwest in MD, more people start commuting to Baltimore instead of Washington. So instead of there being a relatively clean sprawl/rural divide you can point to as a boundary, there is instead a more or less continuous sprawl that transitions from DC suburbs to Baltimore suburbs, and the mixing zone (particularly the Laurel-Columbia belt) is more accurately a suburb of both rather than one or the other.