There were actually many different regional cultures within the South. The Upper South, which included Virginia, was quite different from Lowcountry South Carolina and Georgia, which was in turn very different from the cotton belts in Mississippi and Alabama. More than anything else, slavery made the South unique. By the Civil War, the vast majority of Southern capital was tied up in slavery, even more than in land.
The first development that made the South distinctive was the introduction of African slaves during the colonial period. While slavery existed everywhere in the colonies, they labored in the largest numbers in tobacco and rice plantations in the Chesapeake and Lowcountry, respectively. The second development was the introduction of the cotton gin, which was patented in 1794. The cotton gin made short staple cotton profitable, and resulted in a rapacious appetite among ambitious would-be planters for land and slaves.