The Soviets fell apart into distinct states (Russia, Kazakhstan, etc) because they had been as much under the USSR, albeit a Russian hegemony had reigned with most of the control.
The Soviets weren't very much united in many respects. The former Russian Empire had conquered and subjugated non-Russians in the 'Stans, and those people were under control until the fall of the Soviet Union. So, in more simple terms, these nations that broke away from one another were already nations within a greater nation. So it was already sectionalized.
Correct me if I'm wrong, but while there are ethnicities and distinctions of regions in China, I don't think its so drastic as it was in the USSR nor is there a mass frustration of people distinct from the Chinese in being under Chinese control (save for Tibet and the like).
The Soviets weren't very much united in many respects. The former Russian Empire had conquered and subjugated non-Russians in the 'Stans, and those people were under control until the fall of the Soviet Union. So, in more simple terms, these nations that broke away from one another were already nations within a greater nation. So it was already sectionalized.
Correct me if I'm wrong, but while there are ethnicities and distinctions of regions in China, I don't think its so drastic as it was in the USSR nor is there a mass frustration of people distinct from the Chinese in being under Chinese control (save for Tibet and the like).