I think a large part of it is because MTFs are both more fetishized and more feared. They are subject to our societal fear of amabs but without all of its benefits and they experience the fetishization of women so they become more of a hot button topic. Transphobes aren’t scared of trans men touching their kids because they don’t let men near kids and because they don’t expect afabs to be pedophiles. And with most transphobes being cis men it’s a lot less likely for trans men to be fetishized.

Fair warning I’m not trans so I might not be as knowledgeable as I think