Oh, this is an easy one to answer.
Because women want single sex spaces for their safety, privacy and dignity.
Whereas trans people don't want their own spaces which respect their safety, privacy and dignity, they want women's spaces. They're not interested in spaces which don't have actual women in. Because a changing room without any women in it is just an empty room. It doesn't validate them in their belief that they are the same as women.
If they were campaigning for their own spaces, they would be campaigning for equality and I would support them in that.
But insisting on being in women's spaces - even in the wake of a Supreme Court judgment telling them it's not appropriate for them to be in there - is not about equality, it is about supremacy.
Women aren't non playable characters in trans people's lives. We don't have to accept their needs being placed above our own in the hierarchy of rights.
Believing that male "women" take priority over female women is just misogyny, like every other form of misogyny.
You cannot be a feminist and believe that trans women are women and belong in women's spaces. The two things are incompatible.