Because of Transmisogyny. Trans women or trans feminine people in general are seen as a threat to patriarchy and the gender binary. There’s a lot of material on transmisogyny theory but to put it in very simple terms: There’s a hierarchy where men, through being cis and portraying traditional masculinity are afforded personhood, and everyone else is dehumanized and considered either a human incubator or a freak. Cis women and to some extent, afab trans people are subjugated because of a potential to give offspring but are placed on a higher level then those who deviate from this binary. Trans women especially but others like queer/effeminate men can also be othered into this category. To reinforce the binary and thus men’s role in the hierarchy, transmisogyny is the means to do this. It’s also why terfs are a thing, some women, especially privileged cis women, despite being oppressed by men still want to maintain power over trans women.

Because transphobes

Transphobes think all trans people are scary rapist men looking for a way past the impenetrable magic barrier that only lets women into women’s toilets, and that there’s no reason anyone would want to go into the men’s toilets, so trans men are fictional and don’t need or want any rights, just like the law doesn’t give rights to elves and hobgoblins.

Because there are a ton of laws protecting women’s spaces and rights specifically. Not so for men.