Keep in mind that the two major communist revolutions in the 20th century occurred in regions where the productive forces of capitalism were not fully developed, i.e. China and Russia. The whole point of Marxism is an analysis of history based on antagonism between classes - the bourgeoisie emerged as the dominant ruling class in western Europe via revolutionary struggle against the feudal aristocracy; this is commonly known as the democratic revolution or bourgeois revolution. In Asia, Africa, and Latin America, the march of history skipped from semi-feudalism (feudal economic backwardness altered by the influence of capitalist imperialism) to proletarian revolution, forgoing the initial bourgeois revolution against feudalism. Thus many elements of the anti-monarchist (in the case of the USSR) or anti-colonial (in the case of the PRC) bourgeoisie were integrated into the new proletarian state, leading to the rapid collapse of socialism and rapid restoration of capitalism.

It's also important to remember that class struggle continues after the formation of a socialist state (the notion that a socialist state is the final permanent end-result of proletarian revolution is a distortion of Marxism) this is what happened in China in the 60s as the mass of workers, peasants, students, etc. attempted to spear-head the struggle of transitioning from socialism to communism by rebelling against the bureaucrat-bourgeoisie within the communist party. (The new ruling class comprised of the former intelligentsia)