So ever since the Qin Dynasty, China, or at least its main territories managed to stay unified, with the exception of a few periods in between some dynasties. On the other hand, the territory that consists “Rome” never managed to stay together after the fall of the Roman Empire, with even sub-territories such as Spain and Italy being divided. I recognize that there areas were mountainous, but then again, so was South China. Why was this?
I previously had a stab at a similar question, albeit (ironically) from a Roman perspective. /u/wotan_weevil took a more China-centred approach to another similar question, but more concerned with outer 'imperial' territories like Turkestan.