Why was slavery started?

Answer 1

Cheap labor.

Slavery has existed throughout all of human history, and surely also pre-human history (even ants do it). It did not start in America or Africa; it merely continued.

Industrialization finally ended it, because slaves were found to be inefficient at factory work. Hence the industrialized North was the first to make slavery obsolete... and once that occurred, the North was free to turn slavery into a moral stick with which to flog the pre-industrial South.