Cotton is a way more labour intensive crop. The bolls had to be picked by hand, and because they're mostly air they take up a huge volume, meaning more trips back and forth to empty your basket/sack/whatever. In addition to that, there was a MASSIVE upswing in demand for cotton because of the industrial revolution, and the mechanization of textile manufacturing. Suddenly machines could card, spin and weave cotton faster than a hundred people could. With that skyrocketing demand came the plantations, just like with sugar in the Caribbean; huge monoculture farms planting on an industrial scale. You can pay workers to do that, but buying a slave is an upfront cost equal to about a year's wages for a free worker, and you have that slave until they die. Buy a woman and you can make more slaves for no additional cost.

It's monstrous, but they considered it solely an economical decision.