Not an expert either, but the central plain is also the homeland of what we call the "Han". That wheat growing area is where the first Chinese state emerged and it later expanded into the southern borderlands/hills. Flat plains are easier to control than wild hills.

Basically no overseas Chinese community spoke Mandarin before Singapore mandated it/people fled the revolution, which pretty much says it all. How does it vibe with a "collectivist, interdependent" culture to be the ones chasing $$$ over the ocean while the "individualists" stay home and try to forbid overseas exploring?