Why did fewer cities develop in the South during the 1700s?
The lack of port cities.
In the 1700s, the south had only a few ports, Charleston, Savannah, and New Orleans. And only Charleston saw any degree of shipping.
The north had four large port cities which were all developed in the 1600s, Boston, New York, Philadelphia, and Baltimore. Not only were these ports magnets for shipping, they were each large ship building ports.
The inland cities grew in proximity to these ports, associated rivers and roads.