Because Communists believe in the doctrine of "false consciousness", which ironically was created as a way of explaining why the world didn't turn Communist as Marx predicted.
The original Marx works said that inevitable economic forces would cause the proletariat to rise up and overthrow the bourgeoisie, and that this global revolution was imminent (bear in mind he was writing this in the late 19th century). Not only did this not happen, but the majority of working class people in the West didn't support Communism, or even Socialism in many cases, and this continued to be the case over a century since the Russian Revolution.
This made no sense to vanilla Marxism, as it appeared the proletariat were acting, voting and sometimes even fighting against their own best interests. Why was this? It certainly couldn't be because Marxism is a big sack of balls, definitely not! It must be because ... that's it! Because capitalism has brainwashed everyone into thinking that Communism = genocide and starvation! This was born the doctrine of "false consciousness", found nowhere in Marx's writings themselves, that said that capitalism creates a culture that tricks the working class into supporting it, and it's up to enlightened Communist intellectuals to put everyone straight.
This is why Communists are so insufferably smug and condescending. Because mainstream Communist theory now literally tells them that they have broken free from the matrix and are literally smarter and more enlightened than anyone who disagrees with them who are just brainwashed sheeple in need of a (patronising) education. It's literally the Reddest of Pills.
