How do you explain the communist successes of the twentieth century?

It was not as successful as you think it is. It was only in a limited area while capitalism and democracy flourished everywhere and still flourishes. Communism only exist in Cuba and North Korea, so how can that be a success? China is a capitalistic economy with a Communist government. The collapse of the Eastern Block in the early 1990s caused communism to fail. It was more flawed than capitalism and it showed. The Communist governments just used propaganda to show how their systems were superior.