Why is the original theory of socialism/communism of Karl Marx very unlikely to become the theoretical foundation of any democratic regime?
Nowadays it seems that many young people are embracing Marxism (or some sort of illusion of it). However, it is widely accepted in Western that there are no Marxism-based democratic regimes in the world. To avoid confusion, let us defined a democratic regimes to be the one that is at least labeled "flawed democracy" in the democracy index.
However, the original theory Marxism does not seems to be against democracy. It highly values the rights of working class, criticizes the super-wealthy capitalists, and aims to built a society of equity ("a classless society" in which the state would wither away). The idea of communism might looks very unrealistic, but as far as I remember Karl Marx himself never said how long it would take to establish communism ("all property is communally owned") and never imagined Mao should want to build communism in a couple of year with Big Leap Forward. Marx himself might imagine the communism to be achieved in thousands of years after the whole human sociality becomes highly wealthy through development.
My question only focuses on the original theory of Karl Marx and Friedrich Engels (not limited to The Communist Manifesto). Namely I am asking why the origin theory of socialism/communism invented by them failed to give birth to democracy till today.
To avoid confusion, let me emphasize that I am not asking why the variants of it, namely why Leninism, Stalinism or Maoism failed to establish democracies, because that were how Lenin, Starlin and Mao interpreted the original theory of Marx and Engels. Although they can be used as references to argue the issues with the original Marxism, i.e., what makes Marxism so easily abused by dictators? Or in other words, what nature of socialism/communism due to Marx and Engels makes it unlikely/impossible to become the foundation of democracy? (I believe basically all democracies today are based on the theory of capitalism. For example, the Wealth of Nations)
PS: By "socialism", I am also not asking about the modern Western socialism as shown in say North Europe.