The question really is, "Why doesn't the Bible speak out more strongly against slavery?" Moreover, the Bible appears to not only condemn it, but to support it. Why didn't God come out in no uncertain terms and say, "Thou shalt not commit any form of slavery, it is an abomination." Especially if God, who knows the future, knew that many years later His Word would be used to validate the very cruel and inhumane institution that was slavery in America.
When the Bible mentions slavery, what does it mean? Most of us probably have some sort of image of Kunta Kinte from the movie Roots. That is an inaccurate portrayal of slavery in biblical times. What many fail to understand is that slavery in biblical times was very different from the slavery that was practiced in recent history especially in the early 19th century. The slavery in the Bible was not based exclusively on race. People were not enslaved because of their nationality or the color of their skin. In Bible times, slavery was more a matter of social status. The term was used in a much broader sense back then. A king would refer to his subjects as slaves. People sold themselves as slaves when they could not pay their debts or provide for their families. Some people actually chose to be slaves (indentured servitude) so as to have all their needs provided for by their masters. Many slaves volunteered themselves into servitude (slavery) as a way to earn a living or a trade. Back then societies didn't have the sophisticated form of education or financial practices that we have today, like filing for bankruptcy etc. If you didn't have a viable way to earn a living, or you were in debt, you sold yourself out to another so that you could survive.
Slaves were valued and seen as more than mere property, many times they were seen as members of the household. Many slaves even owned land and property, something that was illegal in the modern version of slavery. Additionally, many slaves in the ancient world, especially those held by the Jews, were able to earn their freedom after a period of time. Although, many people became bond-slaves (pledged to remain in his master's household for life) because their situation was better as a slave than as a free person. We sometimes assume a modern frame of reference when we talk about these things, but one must remember that life was extremely hard during these times, and to be free meant you had no guarantees that you would have enough food to eat or even a decent house to shelter your family.
God provides a realistic approach to the issues of the times. Slavery is definitely not the optimal form of relationship for individuals, but for most of the world's history it is a reality. Therefore, the Bible addresses slavery and provides certain safeguards to make sure that slave are not mistreated and that their humanity will be respected. Scripture gives instructions on how slaves should be treated (Deuteronomy 15; Ephesians 6; Colossians 4), but does not outlaw slavery altogether. In which, many people erroneously see this as the Bible condoning all forms of slavery.
Both the Old and New Testaments condemn the practice of "man-stealing" which is what happened in Africa in the 19th century. Africans were rounded up by slave-hunters, who sold them to slave-traders, who brought them to the New World to work on plantations and farms. This practice is abhorrent to God. In fact, the penalty for such a crime in the Mosaic Law was death: "Anyone who kidnaps another and either sells him or still has him when he is caught must be put to death" (Exodus 21).
Let's remember that just because God regulates a practice that does not necessarily mean He approves of it. Consider the issue of divorce. The Pharisees asked Jesus, "Is it lawful for a man to divorce his wife for any reason at all?" And [Jesus] answered and said, "Have you not read that He who created them from the beginning made them male and female, and said, 'For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh' What therefore God has joined together, let no man separate." They said to Him, "Why then did Moses command to give her a certificate of divorce and send her away?" He said to them, "Because of your hardness of heart Moses permitted you to divorce your wives; but from the beginning it has not been this way." The Pharisees come with a legal question about which regulations ought to govern divorce, and Jesus responds in a very unexpected way, saying there shouldn't be any divorce. The Pharisees are immediately confused. "But how could it be that there shouldn't be any divorce if God regulated it? As with divorce, the same was true for slavery. The rules regulating slavery were added "because the hardness of the hearts". Humanity had created a situation where slavery existed and served certain functions in their societies, "but it was not that way from the beginning." In the beginning, that wasn't God's intent. Once sin enters the world in Genesis chapter 3, everything changed, and the ideal world in which to live was no longer. God, in His kindness decided to regulate the practice, because of the evil that is in man's heart. God made rules to keep the evil of the practice to a minimum. Could God have prevented slavery from ever existing? Yes, of course, just as He could have prevented all suffering, yet He didn't. But just as Joseph said of his own suffering as a slave, "You meant it for evil, but God meant it for good," even slavery can be used by God to accomplish a much greater good.
Lastly, the message of the Bible isn't to reform society. The Bible isn't this big book of rules. Yes, it has rules, but the focus isn't to make people behave properly, it is to point the way of salvation through the savior God has been so kind to provide. When an individual receives freedom from the bondage of sin, and is no longer a slave of sin, their heart becomes changed. God is in the business of reforming, not society, but souls. It is then that people will come to understand that taking advantage of, and abusing another who is made "imago Dei", in the "image of God." God's plan is better than imposing a ton of laws that prohibit slavery which would only serve as to harden people's hearts. No, God works from the inside out. His focus is always on the heart! A person who has truly experienced God's grace will in turn be gracious towards others. That would be the Bible's prescription for ending slavery. As the Apostle Paul wrote, "Faith in Christ Jesus is what makes each of you equal with each other, whether you are a Jew or a Greek, a slave or a free person, a man or a woman." Galatians 3:28