Because in many cases a man who lived as a slave could not stop his master from beating his son or raping his daughter, fatherhood under slavery was denied in a way that

the conditions that made the byzantine empire vulnerable to attacks by the muslin turks were the sack of constantinople by western crusaders, the conquests of the sultanate of rum, presence of rival kingdoms and finally the set up of fiefdoms at the empire which were owned by the ghazis (turkish warriors).

i'm asking the same thing i asked this other cat all of