Not trying to marginalize that the owning of another human being is wrong because it is plain and simple. However, The owning of slaves was very, very expensive. Slave owners were more akin to the 1%ers nowadays. It was similar to purchasing a car nowadays 100% upfront in cash. Not only that but you had to feed, clothe, and house them. Slaves were beaten, yes, but not as widespread as you might believe. If you beat a slave so severely to where they could not work or to the point of death, they were no good in the fields. Sure there were more slaves in the South as at that time we were an agrarian society and as much as 75-80% of the entire country's taxes were paid for by the Southern states at the time of the Civil War(probably the single biggest reason Lincoln wouldn't let the Southern States secede and be on their way but that's an entire different debate). Slavery was heading out with the turn of the industrial revolution anyways as those pesky 1%ers, always looking to make a profit, would have invested in machinery rather than human capital that, again, had to be clothed, fed, and housed thus eating into their margins.
Not even mentioning the fact that less than ~10% of slaves that were brought to the Americas were brought to the colonies and eventual states, the vast, vast majority of them ended up in South America and the Caribbean as to work in the colonies of the European countries.
So many slaves were brought because, if any of you have children and try to get them to do something they really don't want to do, you'll understand this, slaves typically don't make very good workers. Try getting little Jimmy to rake leaves or limb trees and you'll find he's not very efficient unless there is some sort of compensation involved.
Also, slavery is almost as old as prostitution and still exists in this world today as sad as that is to say.