1. Slavery was one way of paying a debt. That exists today in garnishment of wages, for things like monetary debt and child support. People who do not pay child support can go to jail. If you are going to categorically condemn slavery in Scripture, then you need to condemn this also.
2. The Bible explicitly condemns man-stealing, which is punishable by death. Chattel slavery in these United States was based on man-stealing. Furthermore, slavery here was much closer to the enslavement of the Hebrews, which God ended by force.
3. Slavery predated the nation of Israel, and the laws about it in the Old Testament were regulations on an existing practice to ensure good treatment of slaves.
4. Everywhere Christianity spread and took over saw the eventual abolition of slavery, and that movement was led by Christians.
5. In a nation drowning in the blood of 50 million unborn babies, the idea that we are somehow more righteous than ancient Israel is laughable. We are more comparable to the pagan Canaanites who were driven out for their abominable practice of child sacrifice.