I'm reading a book and from what I understand, the CCP and the KMT had the same goal, despite having different philosophies on how to achieve them. They eventually turned against each other and the KMT became the dominant power - until Stalin helped the CCP and Mao Zedong came to power. The book, however, doesn't explain why Stalin chose the CCP over the KMT.
The Soviet Union did help the KMT of Sun Yat-Sen, for example they helped the construction of the Whampoa military academy, a good chunck of both communist and nationalist formed in that academy.
The Soviet Union stopped actively supporting the Republic of China when Chiang Kai-Shiek rose to power, he decided to expel the communist from the party, cut ties with the Soviets and looked to USA and Europe for assistance.
It seems obvious that the reason the USSR preferred the CPC was that the KMT became staunchly anti-communist and pro-western, while the CPC was aligned to Moscow.