Because of their deepseeded feelings of inferiority. They automatically relate to any cause that appears to be in favour of helping weak losers, even it they're not a part of that demographic themselves. Same reason why so many white people are part of nigger lives matter, why so many men are "feminists", etc. All of these leftist movements exist to band together the misfits, weaklings, and losers in a kind or moral crusade against regular humans.