Answer:
Most ancient civilizations practiced slavery in some form. European feudalism was a form of slavery. Some Native American tribes used slaves. Ancient Romans, Greeks, and Egyptians kept slaves and their societies were considered enlightened. The slave trade of Africans began centuries before the Europeans and Americans became involved. I believe that slavery began as a way to utilize captured enemies, followed by slavery becoming entrenched in those societies.
In more modern times, slavery has been outlawed in most places as the recognition of human rights has become accepted. However, even where slavery and trading in humans is outlawed, criminals will still commit whatever crime is accessible to them. Slavery is just one of the many crimes against others. Slavery exists, like any other commodity, because there is always someone who will pay for it.