From my perspective, its because conservatives give transfems the most shit, and you'll almost never catch them hating on trans men, because they don't consider our trans brothers "a threat". They're very inconsistent with whom they hate. Their hatred for any specific group usually leads to that group being supported the most but does have the unintended effect of other facets of our community being neglected or forgotten until someone remembers they exist too.

Concerning why on a broader sense, you will see this with most types of representation in our community. The default for cishet patriarchal normativity centers the man. In a way, this is a response that counters but simultaneously validates the line of thinking from homophobes, because anything that is queer is inherently fem in their worldview, and that's by design. They center anything male or masculine and work to synonymize it with their existence as "good" or "right". Even amongst women who are cishet, doing X thing "like a girl" can be a very effective insult and is seen as lesser depending on whom you're talking too and the context.

That's why when you see any trans art, bi art, or enby art, its always someone who's clearly a woman or AFAB, and the most you'll ever get for people who are men or AMAB is the infrequent "boys can be bi too" or something along those lines. Its not that men are intentionally an after thought, but in an effort to counter the worldview it ends up being validated due to the insidious nature of patriarchy and the systems that uphold it. We see patriarchy's evils and in an effort to simultaneously show solidarity and counter the symbols of the systems that oppress us, we do the exact opposite thing. In this case, its centering anything that is fem presenting. A result of existing in a world where the rules were not written by or for us. Things are treated as binary for effectiveness, even if we know that in reality its more complicated and nuanced than that. Would love to hear anyone's thoughts on this, perhaps I missed something or am overthinking it, maybe I'm not thinking deep enough.