During the time period that you mention, the main area in which slavery expanded was the area we call the “Deep South.”
Before this time, slavery was most common in the Upper South. This all changed, however, with the invention of the cotton gin. With this invention, cotton cultivation became much more of a viable option. The Deep South was an area that was relatively good for cultivating cotton. With invention of the cotton gin, the cultivation of cotton spread over this area. The spread of cotton meant that the demand for slaves had to increase. Slavery expanded in this area, then, because slaves were needed to work on the new cotton plantations.