The Failure of the Marxist Revolutions

No one has achieved communism as we talk about it with a stateless and classless society but why would it happen that quickly again there's this idea that Mao has which i come back to over and over again. The failure of the Marxist revolutions is obvious but it's also expected because revolution isn't an event it's a process in which contradictions are played out on the scale of centuries not decades.