Okay, but you won't like the answer. I'm not getting into the right or wrong discussion.
Cheap labor. There were not enough people living there to perform all of the jobs that needed to be done. Slavery gave the influx of population to have enough workers. And slaves bred more slaves. Simple economics. There were plenty of poor people, living off of the land, small farms and such, but not enough to do all the jobs at a plantation. If we had the immigration of the late 18th and early 19th centuries, at that time, slavery may not have happened, or would have been abolished sooner.