I always found it strange that how fast did the unified China fold. In fact, it feels more similar to the dissolution of the Soviet Union. Does it have something do with the administrative structure of late Qing China or with Western spheres of influence?
There was, in fact, a multi-sided, protracted civil war - the "multiple states" you're talking about were in fact the enclave territories of multiple warlords.
While the Kuomintang eventually suppressed the many warlords of the Warlord Era, that was a process which took seventeen years, between the Kuomintang formation of the Republic of China in 1911 until 1928. Suppressed is a strong word, however, seeing as some of those Warlords were appeased and later rebelled in the Central Plains War of 1930 - some were still in power during the Japanese invasion.
Then we run into the Communists, who had been kicking around basically since the founding of the Republic of China as part of the Kuomintang government. After the "Northern Expedition" (the Kuomintang's war on the Warlords), Chiang Kai-shek, the right wing leader of the Kuomintang, turned on the Communists in his party to strengthen his own position. In 1927 the Red Army was founded, which led to the Chinese Civil War, which wasn't resolved for another 20 years, with the defeat of the Kuomintang.
So China had a much longer period of Civil War than Russia, one that lasted almost forty years.