Marx believed in the natural progression of economic systems, from feudalism to mercantilism, mercantilism to capitalism, and capitalism to… well, something else anyway. Socialism, communism, fascism, and really any of the other isms that came about in the late 19th and 20ths centuries were meant as post-capitalist systems. Marx of course was a proponent of socialism or communism, but it’s not a foregone conclusion that one of those will be the preeminent system after capitalism.

Anyway, my point is that the USSR et. al. were too early to the game. Capitalism hasn’t yet run its course naturally.