Why is slavery so widely used in America?

Slavery had of course been a persistent and contradictory issue since the framing of the US Constitution, having been awkwardly written into it through numerous compromises. The reason 'why' was economic development. Without slavery, the agrarian culture of the southern states would have died out long before 1860. It's commonly taught in early grade school that cotton was king in the south at that time. But slavery was actually on the way out because of the economic forces associated with harvesting cotton---it was hand labor, it was slow and it was losing its profitability. Then at the turn of the 19th century, the Cotton Gin was invented which effectively increased the output efficiency of southern cotton causing the institution of southern slavery to rebound dramatically. Between 1830 to 1850 the output of cotton rose by a factor of 4 using the same slave labor base.