Earlier quoted context omitted.
Well, that goes for every commentary on ancient societies. Unless it's explicitly about slaves and outcasts, it's about the at most 10% of the society that wasn't. At some point the Romans got more people to participate on their society, but that's centuries after any of that.
Do you have a source for this?
It's true. Just read any hisoty source for any historical period before modern times. See how many accounts you can find about the lives of "common people", which usually means slaves, serfs, etc. Even in modern times, you hear about Churchill or Lincoln, but where are the stories of the carpenters, or farmers, the milk maids or the nurses, the midwives or the factory workers? Who remembers their names, what they achieved, what they held dear, who they loved and married, and how they died?
Most of hisotry is about the people that hold all the power in their hands and most of the time their great deeds is to crush everyone else underfoot. We learn the dates and placenames of the great battles where king so-and-so defeated king such-and-such, but do we ever hear the names of the hundreds or thousands of their men who left their families and land to go and die a sad, senseless death?