The question is still a little bit fuzzy, but I think its due to not understanding what drove trade in the pre-industrial era, so I'm just going to get into that.
The basic consideration here is transport. Prior to the invention of the railroad, shipping over water was vastly more efficient than doing it any other way. So back then it was almost more sensible to look at a country's map as its coasts and navigable rivers. Transport and communications any other way was usually so uncompetitive that it barely happened by comparison.
So to be commercially important in the 18th Century, there's one thing a city generally needed to have: access to shipping. Every major city back then was going to either be a coastal port with a good protected harbor, or lacking that at least be sitting near the end of a navigable river. The bigger the navigable watershed drained by that river, the better. New York city for example had a protected harbor on the mouth of the Hudson river, which drains pretty much all of eastern New York state, and with canal building was connected to the entire state, west to the Great Lakes, and north to Montreal.
This was considered so important that early on it was felt that every state required a port. This explains some of the odd little water-seeking panhandles in some of the early states like New Hampshire, Alabama, and Mississippi (and early colonial Pennsylvania). It is also the main reason for the east-west orientation of most Atlantic states, and the north-south orientation of Gulf states.
To go through the southern cities the question called out, Charleston is a natural harbor at the mouth of essentially all the rivers that drain the state of South Carolina (barring the Savannah basin). Richmond is near the mouth of the James, which drains all of central Virginia. Savannah is at the mouth of 2 rivers that drain the NE half of Georgia and the rest of South Carolina. New Orleans is the last (barely) viable place to put a port on the vast Mississippi, draining about a third of the US portion of the continent.
With the advent of the railroads in the 19th century, new cities begun to spring up at interior rail hubs. These include places like Kansas City, Atlanta, and Ft. Worth.
Cities with good ports of course were still important (you can't exactly build a railroad over the Atlantic), but you only need so many of those, so port traffic had a tendency to become consolidated to the larger better-equipped ones like New York, New Orleans, Chicago, and Norfolk etc. Cities on rivers (without ports) were still important, but railways were now where the growth was, and those don't require a river. So while Richmond still grew, its growth was nothing like Atlanta's.