Arguably, that’s because except for Cuba, they stopped being Communist. Now they are more accurately fascist. Looks like fascism won the war after all, or at least finished second to the democracies, with Communism a distant third.
One other point: “fascism” is nothing new. It’s basically hardcore nationalist totalitarianism. Many of the Communist movements even embraced virulent forms of nationalism because nationalism, unlike Communism, is compatible with human nature. Fascism will always be with us because that’s the form of government that we tend to default to when we don’t elect our government and limit its powers. Communism, by contrast, is an idea that was formed in the mind of a few intellectuals and became a big fad, but was totally impossible given the nature of how humans actually function socially. This required arguably more repression than even the worst fascist governments. Nazi Germany probably takes the award for being the scariest ever totalitarian government, but almost all the Communist governments have been more repressive than the more conventional fascist dictatorships of Italy, Spain, or Japan. Because in countries like those, all a citizen had to do was know who was in charge and respect his betters. His obligations were negative: don’t do shit that pisses off the powers that be. The Communist citizen, by contrast, had to study and learn an ideology and always say exactly the right things or else risk “reeducation” and possibly liquidation as an enemy of the people. His obligations were positive: show sufficient zeal or be under suspicion. A society just can’t sustain that kind of idiocy.