[namlem]

Geographic reasons aside, there is the cultural factor of the Mandate of Heaven, which spread throughout China in its early days. The Mandate held that Heaven wants things to be a certain way, culturally, aesthetically, etc., and this is how everyone should be. This created a sort of religious unity over a large geographical area where maintaining certain cultural practices was considered to be a necessity. As a result, when kingdoms fragmented or were conquered by other peoples, their cultures didn't change much and they continued to be similar to one another, making future reunification much easier.

[khuey]

The West had divine right and the Catholic Church. The pope literally crowned kings in the Holy Roman Empire for hundreds of years.

[PeterisP]

However, the principle of splitting the inheritance (including huge tracts of land) among sons is a big cultural difference that makes countries more fragmented.