They act like the ideology is static and everyone agrees with each other and how things were in the past.
They act as if liberals and conservatives weren't lynching minorities a few decades ago, then spend millions of dollars trying to court them to their side today. They act as if they weren't sterilizing you for being disabled or gay for centuries, then spend millions encouraging corporations to throw parades for them. American liberals act as if their best selling broadway show isn't about pro-American POC playing as historical figures who would've put a chain around all of their necks and sent to the fields.
I don't mention these things to defend socialist countries that had the same or similar policies towards minorities. But it's fucking annoying to see that the sentiment that only capitalists are allowed to evolve and change their stances on things. "Stop crying about slavery, that was hundreds of years ago. But every sin under communism must be scrutinized because everyone today still supports the same exact beliefs a hundred years ago."
The proximal reason is because their approach to communism is thought-terminating clichés. It's not coming from a place of understanding or curiosity, it's just regurgitated propaganda that villifies every communist leader and state as a bloodthirsty authoritarian that rules by subjugation and forced uniformity. They tell and retell that story.
The real reason has to do with why they want a thought-terminating cliché. Usually it's just because they're losing an argument. Sometimes it's because anticommunism is useful for left-punching in general, just like villifying the word "socialism". The fundamental goal is to make communism socially unacceptable and police each others' behaviors towards it, which is a legacy of the Red Scare.