Because the idea that a man attracted to trans women could not be straight is transphobic. Trans women are women and a man attracted to women can be straight regardless of whether they’re trans or cis.
I suppose that’s possible, but the lack of clarity on that point still makes me uncomfortable. And there’s nothing wrong with bi dudes liking trans women, obviously. I am a bi dude who happens to be dating multiple trans women.
If it makes you feel better, it’s mostly just loud people on the internet. I’ve never really seen any gatekeeping behavior at any of the many shows I’ve been to.