Most of this has to do with what is known "King Cotton" or the "Cotton Kingdom". Cotton was an extremely valuable product; it helped the North's manufacturing, it helped increase the North's capital from shipping the cotton to Europe, and it is estimated that 1/5 of jobs in Britain at the time were dependent on cotton. Back in the South, the invention of the cotton gin helped increase cotton output astronomically. Cotton picking and cleaning is an extremely timely and laboring process, and the need for cotton was still high, so slavery was the obvious answer. Slaves were also growing in population due to their own reproduction rate, perhaps the only time in history this has occurred. All of this had heavily tied slavery to the Southern economy on the eve of the Civil War.
