Which development led to a sharp rise in the number of slaves imported southern United states?

Before the invention of the cotton gin, it was very labor-intensive to remove the seeds from cotton. This meant that cotton was not a very profitable crop to grow in the United States. However, after the invention of the cotton gin, it became much easier to remove the seeds from cotton, which made cotton a much more profitable crop to grow. This led to an expansion of cotton plantations in the southern United States, and subsequently a sharp rise in the number of slaves imported to work on these plantations.