Puritan Board Post-Graduate
The same could be said as to "why would we pick out the command to not sow two kinds of seed, or wear mixed fabrics, and abrogate that?" The scripture does not clearly state that as ceremonial.
You need to understand the distinction between moral, ceremonial, and positive law.
Perhaps instead of reading slavery in the Bible anachronistically, you should actually do a study of what the Bible deems as slavery. The Bible condemns chattel slavery and man stealing. What slavery you see in Scripture is not as you are trying to push it to be.What then would you say of our Scriptural, God-given right to own slaves?