A 14-mile wide stretch of land separates Chesapeake Bay from the Delaware. The canal connecting the two was built in 1829 and Philadelphia/Baltimore remained America's second and third largest cities into the second half of the 19th century.
The region connects two major rivers to the Atlantic and lies in the middle of the most populated corridor of both colonial America and the modern US. Being close to water, it must experience somewhat milder weather than most of the region, and support agriculture, since that seems to be the primary business these days. I'm not sure how long Delaware's loose tax and corporation policies have been around, but the state continues to impose some of the lowest tax rates in the country.
Despite this, the area around the canal remains virtually uninhabited and as far as I can see, has always been that way. The nearest real city is 15 miles away in fairly run-down Wilmington, DE, whose population topped out around 110k in the mid 20th century. New York and Boston both sit on the atlantic coast, yet further south, all the mid-atlantic metros all seem to have developed inland and upriver rather than the atlantic coast or the well-connected bays. Any good reasons for that?
The mere possibility of port facilities doesn't create a city. The port needs to be well connected (by rivers or rail) to inland areas that produce what's to be shipped out, or that consume what's being shipped in—preferably both. Another good predictor of city creation is what's called an entrepôt, a location where cargo must be shifted from one conveyance to another. With the labor cost of doing so already a given, perhaps assumed by others, those are good places to set up manufacturing and warehousing and other enterprises to add value to the cargos. Finally, a place near the fall line offers yet another advantage: lots of water power to run the factories located just upstream from the furthest point that oceangoing vessels can reach.