It's true, China went through a ton of unification -> division -> reunification phases in history. There's even a famous quote for this: "what is long divided must unite, what is long united must divide"

I think one possible reason is that the Qin Dynasty really managed to assimilate everyone into the same shared values, religion, language, writing, and so on. Other empires didn't succeed to that level, and the people in them always had strong differences, language, values, religion, beliefs, writing, philosophy, and so on.