China fell apart many times, but each time the two main parts, the areas around the Yellow River (Huang He) and the Yangtze, were reunited. Why was this? When the Roman Empire fell apart, there were indeed attempts to reunite it, but after the rise of Islam, the northern and southern shores of the Mediterranean were never again ruled by the same empire. The areas around the two main rivers in China, however, were united, and then again divided and reunited, several times. What explains the fact that it was relatively easy to keep the two areas under the same empire?
The fact that the two river basins in China just have a large lowland plain between them that it's easy to march across, while Europe had major seas and mountain ranges that aren't easy to cross and are much easier to defend from.