White slavery doesn’t get a lot of attention because the practice is so archaic. It really only existed back when white people knew the difference between their different ethnic background, and when racial differences actually existed in Europe. They are still there, but they aren’t really an issue now that most Europeans know what black people are.
I don’t know how much ethnic and racial differences actually mattered for white slavery. As far as I know, white people enslaving other white people was a result of war. If you lost a war, your people deserved to be enslaved. The Romans enslaved all the white people they could, from all over Europe and the Ancient Near East simply because they defeated them in battle.