As someone smarter than myself has pointed out in the past - why would God ever be straightforward and clear about the command "do not eat shellfish" or "do not wear garments of mixed fabrics", and yet be so unclear about the wrongs involved in owning other people as property? Why couldn't God declare "do not own other people as property" as easily as He told people not to eat shellfish? You honestly cannot have a reasonable justification for this.
"God's word" literally contains prescriptions for the conditions under which it is allowable to own other people. And now we realize it is never allowable to own other people. And so... even if Christians were instrumental in abolishing slavery (HIGHLY debatable - as it was also Christians who utilized these very passages from The Bible as justification to keep slavery in place), they are going against the prescriptions for slavery laid out in The Bible in order to do so. A Christian protesting against slavery in every form is literally protesting against prescriptions present in their precious Bible. No question.
I believe there is a reasonable answer. To start with the God of the Bible never sanctioned or justified "owning" another person, treating people harshly, or like property. The scriptures indicate that God created human beings in His image. So every person is valuable and responsible to God for their behavior and treatment toward others. He gave the commands to ..
.
love your neighbor as yourself (Lev. 19:18) and
...the stranger who dwells among you shall be to you as one born among you, and you shall love him as yourself; for you were strangers in the land of Egypt: I am the Lord your God (Lev. 19:34).
Commandments as these leave no room for the slavery you are talking about and are unacceptable by biblical standards.
I think you need to realize that what you are calling conditions for "slavery" in the Bible is not a reference to the kind of slavery we think of which took place here in the U.S. or elsewhere when one race was stolen and enslaved by another. The term slave in the Bible means more accurately bondservant or indentured servant. It was economic-based slavery (servanthood). These were people who chose to sell themselves as slaves when they could not pay their debts and they were paid something and/or provided for. This was a common practice in ancient cultures. God's rules on how these slaves (servants) were to be treated show that He cared and regulated the practice so that those in these servant positions were treated fairly and as equals. This is consistent in the OT and NT. God's rules do not in anyway endorse the practice of wicked slavery you are referring to and is in fact a violation of God's command to "love your neighbor". Slave traders and/or man-stealers are in opposition to God (1 Timothy 1:10). As would be any "Christians" who twist scriptures for their own greed, control. unloving treatment of others.
All the detailed commandments, such as the one about shellfish were given to set the nation of Israel apart from the surrounding nations which were immersed in sinful practices, very likely including abusive slavery and much worse. God's command to
love your neighbor as yourself and to treat others with respect and dignity was unique and meant to be a light and a testimony to Israel and the other nations that God placed value on every life, unlike the human thinking of the day.
I think it is also important to note that the purpose of the Bible is not to reform society, but to point the way to salvation. God could have made more and more rules, but unless a person's heart and attitude is changed from the inside by grace and love, rules never accomplish real change. Just like the scriptures point the way to find freedom from the slavery of sin, so freedom from sin results in a changes throughout a society where love for others replaces abuse.