Older forms of oppression have always existed. Look at other species to see that even without a culture or society, the weak are exploited.
The point, though, is that Roman slavery wasn't very different from modern slavery. They had all the key elements we do - the tools to both imprison humans en masse and exploit natural resources, a standardized transportation network to move both troops to where the exploitation happened and and remove goods from where the exploitation happened, literacy and bureaucracy to manage large-scale, ongoing enterprises, etc. And of course, they had their moral rationalizations for brutality, especially brutality that happened somewhere out of sight.
Without metal, without roads, without heavy transport, without structured military units, without literacy, the tools for mass enslavement really don't exist.