Cotton, of course, became the king of the crops that underpinned the South’s economy. Cotton wasn’t the cause of slavery, slavery came first, but with the development of the cotton gin it made slavery favorable as a low-cost method of harvest. This then was a major part of the South remaining a relatively backward, agrarian region. While several of the Southern states were among the most wealthy, that wealth remained with the slave owners and was used by them to expand their operations instead of finding its way into social growth and development.
Cotton also meant faster soil erosion, so the South’s insistence on westward expansion can also be seen in that light. There is far more to this than can be added here in a few words. But I wonder what Henry Clay, a slave owner, thought of his nephew’s position. I’ve not heard.
He didn't have him over for Christmas punch in 1865. Nascent abolitionist sentiment was fairly widespread in the South by 1860. Not dominant, not nearly a plurality, but the snowball had begun to roll down the hill.
The real unasked question in all this is where did antislavery sentiment come from? The answer was British social Christianity, the origin of Methodism, the Salvation Army, temperance and abolitionism. The British sent warships to disrupt the slave trade out of religous sentiment, not for imperial or commercial gain. That slavery was immoral, "wrong", had never been so strongly espoused or defended as in Victorian England and New England.