Because the Lemmy software was developed by communist-leaning developers and they have, prior to the reddit exodus, had the biggest communities. As for the transgender related content, the transgender community was one of the largest to leave reddit for Lemmy.
And if you're transgender and your choices for typical ideology for cisgender community members are communist or fascist, the choice is generally going to be communist.
The particular "communists" in question on Lemmy are at worst naive idealists. They're not calling for fundamental human rights being stripped away or genocide. They're egalitarians -- regardless of if you believe them to be misguided or not.
At their core the "communists" on here are for the most part far better human beings than the fascists. I'd rather deal with their talk of "class warfare", "eating the rich", and hatred of capitalism than hearing about how queer people should be killed.
I think forty-seven people would disagree that mods are to blame. That said, I didn't see your comment before it was removed, so downvotes are my only evidence.