I guess it's just due to the extremely negative connotations the term unfortunately has, largely due to being associated by those on the Right. Which is unfortunate because there's many serious issues affecting men and boys that for much too long have been neglected or even outright ignored. Like how badly boys have been doing in schools and face bias, male victims of abuse, violence, rape, sexual harassment/assault, etc. not having enough aid and also not being recognized enough and there being no campaigns to also end violence against men. Also the lack of abuse shelters and health clinics for men, lack of due process to crack down on false accusations, ensuring the safety and well-being of men and boys just as much as women and girls, etc.

It's a shame because of the negative connotations the term men's rights brings up that people shun the idea of providing aid to men and bringing awareness to things that affect them, and are also quick to associate it with right-wing extremism. But to be honest, I find I don't like the terms for either men's or women's rights, the latter of which has also taken on a bit of a supremacist leaning in more recent times. I think I just prefer the term people's rights, which sounds more all-encompassing and also better gets across wanting to advocate for men and women equally, and bring attention to where both are affected by inequalities or are having their rights infringed upon. We want issues that affect men and boys to be acknowledged, aided and rectified just as much as those affecting women and girls, and it's long overdue for that to finally happen.