The classic question
China was also diversed in terms of nations, languages,... as Europe. But why one China was formed, while one Europe never became in realm.
The question reminds me two other questions:
1. Why has Vietnam always struggled with the idea of unification and the real force of division ?
2. Why didnt China become a capitalist society under Sung dinasty when she had all the conditions for it?
There are various answers:
1. In China, the threat was unidirectional from the northern nomads. In Europe, the threats were multitasking.
2. In China, the bureaucracy was developed to completion as early as in the Qin dinasty.
Personally, I think these two answers are not convincing (I can analyze later the counter arguments). There must be something to do with the ideology of a unified China formed by Suma Tien and Han Wuti.