It’s the geography in large part — North America has the Great Plains running from north to south, Asia has the steppe running from east to west. And during the 100% natural climate cycle, there’s better conditions for at least a few hundred years, then a rapid falloff, producing pulses of immigration outward to the east, west, and south.
North America has the Great Plains running from north to south
How dare you, you geographic determinist! LOL
I was just yesterday working w/ a student on the 30's Dust Bowl -- I had forgotten the shape of the drought map, showing precisely your description of the N/S axis of the Plains:
