Jared Diamond attempted to explain this in Guns, Germs and Steel (which I highly recommend as a book). Essentially, if you look at a map of Europe, you will see it has a very irregular coastline with lots of peninsulas and islands. This means that ethnic differences are amplified as people are isolated from one another (Spain and Italy had large numbers of non-Indo-European ethnic groups well into the early days of the Roman Empire), and so are much harder to unite under the rule of one group of people. East Asia, however, has a very round and regular coastline, with the exception of Korea and Japan which maintain independence to this day, and Taiwan which only became Chinese during the 17th Century, and so it was easier to unite different disparate ethnic groups into one country.

Hey so /u/fishstickuffs and a couple of other users did a great job answering this question a little while back here. You might also want to check out this discussion, comparing notions of continuity. And as luck would have it, today we are hosting an AMA on Tang Dynasty China. You might try over there if there's anything else you're hunting for.

As I learned it, it also had a lot to do with the domestication of rice. Rice is a very high density crop; a few dozen people can grow enough to feed 1000, far more than any of the cereal grains that could grow in Europe. As a result, a lot more people did not need to practice sustenance agriculture, which led to a large bureaucracy that could handle a large polity.

Andrew Marr's history of the world cites a few factors for the continued unity of China leaving the classical period whereas the Western Roman Empire fell and wasn't ever reunited.

Firstly, geography: the Roman Empire had a central sea which facilitated trade but also allowed invaders to move extremely quickly. Wheras the European portion of their Empire was divided by mountains and rivers cris-crossing the Empire in different directions. Geographically it had so many natural obstacles it meant that it was more "naturally" divided by these natural boundaries.

China also historically was better able to repel or at least absorb nomadic horsemen from the steppes (until at least the Mongol counter-example) wheras Europe kept having wave after wave of population migration from the east which constantly meant a state of turmoil.

Also, he cites Monotheism as being a big guilty party. The chinese weren't divided into "believers" and "non-believers" or schismatic sects and so was able to maintain a unified society.

Finally, China kept a strong centralized control over it's military forces, meaning they didn't suffer from the late-Roman tactic of increasingly relying on Barbarian auxilliaries to fight other Barbarians. So they were able to keep their military at a better quality than the declining usefulness of even Rome's own forces as time went on and training and discipline decreased.

Andrew Marr, The History of The World, p.p. 114-115