Because JK Rowling is taking part in the abuse against trans people. I don't give a flying shit if she has good points or whatever people defending Joanne have to say. By trying to discredit the movement, its beliefs, its people, and supporting an organization that is actively transphobic, JK Rowling is contributing to the consistent violence and discrimination trans people face on a daily basis solely for wanting to live their lives. Rowling's actions means she has blood on her hands, and Redmayne coming to her defense will only make things worse. And really, Redmayne having trans friends does not give him the right to defend transphobia or TERFs. That's the equivalent of the whole "I have a Black friend" defense used whenever somebody says something racist.
Obviously calling out Joanne isn't going to magically make transphobia go away. But her words have actions and repercussions, and if she's going to say stuff that's hurtful or perpetuates a stigma, she deserves to earn some backlash. And I know people here will read this and just go into the usual "wahhhhhhhhhhh Twitter! wahhhhhhhhh cancel culture!" whenever somebody calls out transphobia online. But really, JK Rowling's comments given to her are nothing to what trans people face, and she has to face her consequences, just like you and me. Her writing fantasy novels does not make her exempt.
And honestly, between a woman who has a net worth of at least $650 million and a minority group that sees one in four face an assault and employee discrimination, as well as one in five currently on the streets, all of which to say nothing about the endless amount of discrimination solely because people are feeling comfortable in the gender identity they feel most complete in...yeah, Joanne's backlash is nothing.