The Southern plantation owners believed that the Southern economy depended upon slavery. The rich soil of the South made it rich in agricultural exports like indigo, rice, and tabacco. Cotton grew well also, but it was difficult and time consuming to separate the seeds from the fiber.
Then in 1793 Eli Whitney invented the cotton gin which speeded up the process. Plantation owners saw this as a way to increase their economy. The faster the cotton gin worked, the more manual labor was necessary to plant and harvest the cotton. This meant more slaves were needed and this increased the slave trade.
By 1860 more than half of the American exports was cotton. The South was pretty dependent upon their slaves to continue making the profits they had seen. The more cotton they sold, the less willing they were to discuss freeing the slaves.