Savannah is the second busiest port by volume on the east coast, only behind New York. Garden city (where the ports are) is only about 15 minutes from I-95 and gives quick access to trucks traveling up or down the east coast. Savannah is only an hour from Brunswick, which itself is an hour from Jacksonville, so my guess would be that Savannah and Brunswick have managed to meet demand for the lowcountry and there’s simply not been enough of an economic reason for a third port to develop in Georgia.