If I had to guess, in their minds, because Communists 100 years ago, weren't exactly "progressive" on LGBTQIA+ issues by contemporary standards (despite being well ahead of their bourgeois contemporaries), communists today must be exactly the same way. To them, Communism is this inflexible and unchanging dogma and not the flexible and evolving political movement of the international proletariat against the bourgeoisie.

Because they haven't examined that idea at all.

I wonder how these people would react to finding out that Cuba recently instituted stronger trans protections than the US has

Liberals think political movements manifest whatever reality they imagine is the end goal, rather than depending on the circumstances of the time and situation of what generated the political movement. They think enough people clamoring for communism would quite literally conjure a cartoonishly evil dictator who would institute the American conception of what the 1920s USSR was.

It's purely magical thinking, also liberals view politics as completely external. It's sports or a TV show. They think of politics as praying to various polytheistic gods who manifest their powers based on how many followers they have. Liberals have concluded the communist god is transphobic and would have goals unrelated to its followers' desires.

Part of this might be that there's been a lack of a coherent leftist movement in the west for the past 60 years, so liberals can describe communists by way of cartoonish propaganda.

They recognise communism has something to do with people power and they, as one of a people, equate communism with their own fantasies of gaining power.