Some because they owed a debt they could not repay,some because they were sold by their own family members into servitude for cash/trade.some because of the rich/well to do needed servants and they paid what was considered a good amount of money at the time(not condoning the practice but it is part of history).some because at the time they had no way of defending themselves nor would they. There are many many reasons why but none can excuse the practice.as the bible says never be a borrower nor a lender be,because basically you are a slave to that person. And this is how the slavers trapped the black slaves into staying slaves. They claimed because they bought them that they had to work off their debt which the slaver made sure they could never do because they would owe for food and lodging and could never make up the difference.but the fact is they should have never been made into slaves when no one can own another unless they actually incur a debt.