I think finding a clear answer to this question is going to be difficult, but there are plenty of theories that might interest you. There is only one I’m familiar with from a class I took on Chinese history.

One of the main differences between the Roman Empire and its Chinese equivalent around the same time (Han Dynasty) was that Romans were always minorities in their own Empire. “Ethnic” Romans comprised much less than half of the total population of the Empire, falling as low as 10% around the Empire’s height.

This is in contrast to most of the Chinese dynasties, which were usually Han-majority. There are some examples of ruling classes of other ethnicities (the Mongol Yuan Dynasty, usually considered illegitimate and they used mostly Han officials anyway, and the Manchu Qing Dynasty), but China has always been mostly Han. Today, China is something like 90% Han, and historically these numbers have always been heavily slanted in favor of the Han people.

It makes sense, then, that the Han Chinese people were united so often, since they comprised most of the population throughout most of history. The Romans would have had a hard time reuniting their war-torn Empire since they were such a small part of the population to begin with. There were some attempts, obviously, such as the Eastern Roman Emperor Justinian who reconquered most of the Western Empire before being halted by an outbreak of the plague which killed tens of millions of people. I’m not sure anyone would consider him “Roman”, though. He was probably linguistically and culturally Greek (don’t quote me on that).

Hoping for a more enlightened answer. It’s quite an interesting topic.