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.
Professional armies weren’t the norm, but a general had to deal with the logistics of the army they had.
Knights could ride horses, but they generally need at least two warhorses along with a support train of staff to support the knight in battle. That staff isn’t all riding on horses. You also need a supply network to supply not only the knight, but the support staff for the knight.
In contrast, a steppe army is going be a lot more mobile due to the culture and supply demands. A steppe army is going to be a lot more mobile.
See, I meant it when I said it might have been part of it. If you’re always migrating anyway army life probably was less of a departure. Sometimes that goes in a “they can live completely off the land” direction, though, which isn’t true when you have the concentration of people necessary to win a battle.
I’m guessing that’s true, but I imagine pack animals were heavily used for both passengers and cargo. In Roman times they relied on giant breeds of mule for this.
I should say I honestly don’t know if the “knight in shining armour” of Victorian fantasy ever saw combat that far east. But, even if it was a common style of combat, “the locals are never stupid” and would be capable of traveling light if it made sense. It would take a bit more training than it would for the nomads, is all.