Before the Industrial Revolutions in Britain and the United States caused demand for cotton to skyrocket, there was a pretty limited market for that cash crop. What's more, before the invention of the Cotton Gin, it was painstaking to cultivate, harvest and process, making it even less attractive to prospective plantation owners. This is one reason why tobacco, rice and indigo dominated the plantation agriculture of the southern Atlantic states. To process a single pound of cotton lint, a slave would have to work ten hours, pulling the seed from each cotton ball, one at a time. The Cotton Gin, by contrast, could produce a thousand pounds per day.
As the new Gin became more popular and America expanded westward, more land was converted to cotton production and profits for plantation owners soared.
Ironically, Whitney was an abolitionist who thought that with technology, he might be able to make slavery obsolete and it would die out on its own. Despite his good intentions, the plan backfired and the number of slaves in the United States rose several fold by 1860, along with the price of an African male.