History

If you read Fannon's Wretched of the Earth, what everyone should read after Settlers, also Lenin's National Question both go through the reasoning why.

A small tldr is colonized have to feel a connection to their homeland as a precondition to any sort of successful movement to wrest control of themselves, their lands from colonial imperialist powers, this would be broad nationalism the precursor to internationalism, not to be confused with narrow chauvinistic nationalism, the fasc type we have in the US. You have to move out of this stage and form a socialism movement quick, or you'll get stuck in it and bad shit will happen.

Pretty much every 'third world' country outside the imperial core that has been successful in any form in the past with socialism has gone through that broad national movement with a national bourgeoisie. At first, the national bourgeois allows a colonial nation breathing room from the imperialists for nation building that allows the colonized to explore their own interests, unity, and all that. However, once this is achieved the people need to quickly put down the national bourgeois like the rabid dogs they are cuz if you don't they'll split your nation up, sell everyone and their dad out to the imperialists and turn full murderous fasc (this has happened in LatAm and Africa for failures), and turn to a socialist movement. Socialism or Barbarism basically. This includes CN, pretty much what the KMT did. They were based anti-imperialist early on, whichd allowed CN to grow and fight off the imperialists, but then they started to show their fasc character as conditions solidified and had to be put down to allow for the CPC to flourish.

USSR backed them due to the National question and a issue of survival, it wasn't ideal of course.

KMT was primarily pro national bourgeois and didn't really give a shit about the proletarian or class struggle.