Maybe the classical era too, I don’t know where the start year should be. It ends in the early modern period when bordering agriculturalists like the Russians start expanding.

In other places and times agriculturalists tend to displace nomads on arable land, probably because crop farming can support a lot more people (and therefore fighters) per area.

Any explanation needs to be valid across the whole period and rely on things the nomads had that the farmers didn’t. Horse archery was not new by this period.