Why would ending slavery not count as moral progress according to ethical relativism?

According to ethical relativism, there are no objective, culture-neutral standards by which to judge the moral practices of any society. Because the doctrine holds that moral right and wrong are defined solely by the social codes of a specific culture, there is no external or universal benchmark to determine that one set of values is superior to another.1

Consequently, ending slavery would not be considered moral progress under this framework. Instead, it would be viewed merely as the replacement of one set of social standards with another. Because ethical relativism denies the existence of an objective moral truth, it deprives us of the ability to claim that a society has "improved" its morals; it can only be said that the society has changed its customs. From this perspective, if a society once approved of slavery, that practice was "right" for that society at that time, and a subsequent shift away from slavery is simply a change in preference rather than an advancement toward a more moral state.1