Why did many people continue to accept slavery, even though it was morally wrong?

Slavery was generally accepted by the majority in some societies - if ethics is a matter of public opinion (Cultural Ethical Relativism) then some would say that slavery was ethically OK in those societies where it was the cultural norm. This sort of argument is a key reason why many people oppose CER.

shang dynasty

western zhou (ca. 1046-771 bce)

eastern zhou (ca. 771-256 bce)