One of my guesses (without any support) is, the written language of Chinese played a very important role in keeping China united as a country in the last 1000 years, although it separates, and then united. Why?
Unlike languages that records pronunciation, like English, Chinese characters is basically a “picture” represents the meaning. Look at the illustration in this blog: Chinese Characters.
No matter how you want to read it, the written language is always the same across China. A simple (over simplified) example are the number one, two, three in Chinese: 一. 二. 三. If you have Chinese system in your computer and can read Chinese, you will find one stroke means 1, two strokes means 2, and three vertical strokes means 3. (Of cause, 1000 strokes does not mean 1000). It is just like the Rome representation: I, II, III, IV, V, or the numerical representation: 1, 2, 3, 4… In different countries, they are read differently, but the meaning are the same.
Just like Arabic numbers keeps most of the human on the same page (unfortunately, there are just few signs in common), Chinese languages acted as a glue to keep the vast area together, and they can communicate with each other.
One example is, if Chinese people start to use Pinyin, or whatever representation of the pronunciation of the Chinese characters, within 50 years, there will be at least 100 different languages in China.
In Beijing, 谢谢你 “Thank you”, will be written as Xie Xie Ni. In Taiwan, it will be written as: ㄒㄝ ㄒㄝ ㄋㄧ, and in Shanghai, it may be written as Xia Xia Nong. I can imagine in most of the provinces, or even cities, they can be written differently, if according to pronunciation…
If that does happen, when a group of people cannot communicate with another group of people for too long, China will become an Europe of today.
That is just my guess. Any support to this idea?