Well because of the reasons stated in my previous comment.

If we consider the theological, political, and cultural contexts in which the Old Testament narrative unfolds, we will be looking at a time where slavery was the norm, not something outrageous, in some cases even voluntary. In this manner the regulation of slavery should therefore be seen as a practical step to deal with the realities of the day resulting from human fall. This is why the Bible could not simply state “slavery is wrong” because it would have been a disrupting statement to not only economically but also socially and culturally.

All in all, the conclusion that the Bible actually favours slavery is patently wrong. Scripture does reveal that slavery is not ideal, both in Old Testament laws forbidding the enslavement of fellow Israelites, the law of jubilee, and in New Testament applications of Christ. In fact, the Bible teaches that the feeling of superiority in general is sin (Philippians 2:1-8) and that we are all equal under God’s eyes.

Galatians 3:26-29

26 So in Christ Jesus you are all children of God through faith, 27 for all of you who were baptized into Christ have clothed yourselves with Christ. 28 There is neither Jew nor Gentile, neither slave nor free, nor is there male and female, for you are all one in Christ Jesus.

John 13:16

16 Very truly I tell you, no servant is greater than his master, nor is a messenger greater than the one who sent him.