Indeed trade links relevant, so navigable rivers played a big role - before railways, our main transport was either boats or horses (or camels). Horses needed a lot of grass, which thrives in drier mid-continental climates where trees don't survive wildfires. For example the Mongol empire was good at trade and connecting cultures, covered a huge area, but not (for long) near coasts, and still demanded intense tribal loyalty (elements of such culture was absorbed by the next empire which gradually pushed it back...).