“Involuntariness” is the usual answer.

Indeed, classical liberalism takes the most basic framing of a social question as: “consent or coercion?” In this view, democracy is characterized as government “with the consent of the governed” so slavery and non-democratic government were both condemned for the lack of consent.

This common condemnation of slavery on the basis of involuntariness has caused a large amount of intellectual history to just go “down the memory hole.” Those who routinely condemn involuntary slavery have either forgotten or never knew that from Antiquity down almost to the present there have always been those pro-slavery writers who: (1) presented a defense of slavery based on consent or contract, and (2) interpreted much of historical slavery as being based on implicit or explicit contracts.

My focus here is not on (2), the empirical question of whether or not any historical slavery could be interpreted as being voluntary, but (1), the fact of intellectual history that so many classical authorities defended slavery if based on consent.