That's because the Mail wanted to use this as something else to beat Labour with. The mail obviously don't give a fuck about women's rights. But this issue was an easy win because it ties into their readership's anti woke agenda.
However labour aren't acting like the right want them to (at long fucking last) so the right wing press are scrabbling around for some ways to still use this to damage Labour. If it was a Tory MP saying that trans women shouldn't use men's toilets it would be front page news tomorrow however I guarantee this won't be the case.