As someone with an NPD “friend” - I have to look beyond the insults and threats and see the insecurities and vulnerabilities behind them. Most people just can’t or don’t want to do that and will be insulted if not scared away by the things my friend says to them. There’s also a stigma associated with being friends with someone who is abusive - I keep the friendship secret from all but my closest friends, who have a hard time understanding it themselves.
3 explanations, in order from what I believe most likely to least:
It could be selection bias. All communist nations originated from dictatorships, and as democracy isn’t a key part of communism, any democratic ideas get kicked to the side. It may require a dictatorship in the first place for a communist revolution to occur, as democracy may lead to people feeling content enough with the system that they may not feel it needs fundamental change.
The inevitable need for concentration of power in the hands of a few. Assume that the powerful will always try to concentrate power in their own hands one way or another. Capitalist societies use wealth (a.k.a. purchasing power) to replace the concentration of political power that a dictator would enjoy. As communist societies lack such a mechanism, the powers-that-be can only use political power to force their own superiority.
The centralization of economics leads to concentration of economic power that can be used effectively to buy loyalty from would-be challengers to a dictator’s power.