Because slave owners enjoyed an economic advantage that they were unwilling to lose, they used any number of justifications to defend slavery. And, because the Bible was the most pervasive influence in early America, it’s no surprise it was argued to be supportive of slavery by those who benefited from the practice.
However, it’s worth noting that Christians actually led the way in the abolition of slavery, both in England and America. Christians strongly and publicly pronounced slavery as immoral and incompatible with Christian practice. Examples include William Wilberforce, John Wesley, and John Brown (who went to his death fighting to end slavery in the United States).