Why this should be I’m not sure – I’m not sure that anyone knows – but I do have one or two suggestions.
Whereas most other great civilizations have been centred on large river valleys, this is not so for the West. China has the great Yellow River and the Yangtze; India has the Indus and the Ganges; Egypt has the Nile and Mesopotamia has the Euphrates and the Tigris. All these rivers flowed through well-watered and immensely fertile valleys, which were able to sustain highly productive agriculture and dense populations. Such populations are easy to tax, and powerful and sophisticated states were able to grow, drawing on the economic (and therefore military) capabilities that such population concentrations gave them. These states nurtured the kind of advanced urban cultures which we call civilizations (I have written about this is an essay on the origins of civilization).
Western civilization, on the other hand, did not originate in a great river valley. It grew up on the shores of the Mediterranean Sea, and later in the plains and hills of Europe. There was no very populated region upon which an advanced state system could be based. (In this it was typical of secondary civilizations which are derivative from previous ones – so, just as Japan and Korea are derivative of China, so the West is derivative of the ancient Middle East; however, of all secondary civilizations in world history, only the West broke free of its parent to become a truly original civilization in its own right – but that’s a topic for another day!)
This begs the question, what about the state systems of Greece and Rome? Were they not advanced?
Well, for a start, the cultures of ancient Greece and Rome had their roots in just such river-valley civilizations, as I have said above. In fact, though, for most of the history of ancient Greece, Greek society was based on tiny, comparatively simple city-states; otherwise democracy, which to the Greeks meant direct participation by all citizens in office-holding and decision-making, could not have developed.
In the wake of Alexander the Great’s conquests large and powerful semi-Greek (“Hellenistic”) kingdoms did emerge, but these were based on the river valleys of Egypt and Mesopotamia, and drew on the governing techniques of preceding civilizations.
As for Rome, this too began as a simple Greek-style city-state; it was this very simplicity which caused all the problems when it found itself the possessor of far-flung territories, in the period of the late Republic. However, the Roman empire which arose out of the ashes of the Republic was indeed one of the most sophisticated states in the ancient world – but it drew on bureaucratic traditions inherited from the Hellenistic kingdoms of the Middle East.
My main point, though, is that Greece and Roman civilization did not have a region of super-dense population on which to build; and this left it much more vulnerable to state-destroying shocks than civilizations which did contain such regions.
The internal instability and external invasions of the 5th century administered a profound shock to Graeco-Roman civilization. For me, it is a key point that the fall of the western Roman empire led to a serious collapse in material culture, far more dramatic than occurred in the centuries of Chinese history following the fall of the Han empire. The internal instability and external invasions were just as severe here as for Rome, but while material culture undoubtedly suffered greatly, a whole urban-based way of life was not swept away in the way it was in western Europe after the fall of Rome.
Why was this? In my opinion it was because, in Europe, there was no region of super-dense population, with its potential for tax revenues, which could maintain a sophisticated state in such difficult times. It is noteworthy that, from the mid-5th century, the lifeline across the Mediterranean to the bread baskets of the Middle East was severed (or at least very badly disrupted). European society was forced to return to a simpler, more rural level. In China, on the other hand, the Yellow River valley continued to sustain its dense population through these dark centuries, and this allowed the bureaucracy, developed by the Han empire, to continue to function, even though at a more basic level.
So, western civilization received its first great shock after the end of the Roman empire in the west, whereas China did not after the fall of the Han. This first shock led directly to the next great shock – but more on that in another blog.