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.
You’re right that horse archery wasn’t new but the bow used by the Mongols was more advanced than it’s contemporaries. It was much smaller and allowed for better freedom of movement for the archer without sacrificing power. There are a lot of other reasons why they were able to dominate the known world the way they did and I believe John Green has a whole Crash Course episode devoted to them and explaining why they are the exception to so many historical trends which is worth checking out
It’s been a hot minute(15+ years probably) since I studied the Mongolian empire at all so anyone feel free to correct me if I’m mistaken but that was my understanding
It’s not just the Mongols, though. The Golden Horde was a thing a full century later, and even when the nomads were too busy with infighting to expand into Europe, China or Persia they kept the farmers out of their home steppe.
I’ll watch John Green, but I’m pretty skeptical of the whole bow theory for that and other reasons.