It's because of the concept of "intersectionality", which basically stands for the proposition that all prosecutions of groups come from the same place, and therefore all "persecuted peoples" must support each other. This concept is accepted as an obvious fact by many on the Left, and is the reason that supporters of one downtrodden group (say, African-Americans) will automatically support another group they see as "persecuted" (such as "Palestinians").

The idea is patently nonsensical to anyone who is intellectually honest with himself, but holds an appeal to anyone just looking for something facile that "sounds good".

I read an editorial recently (I think on ToI) that points out that the Women's March platform doesn't mention much of anything to do with women's rights. It's virtue signaling.

the organizers of the women's march are promoting hate and discrimination. their primary target is men in general, and white men in particular. like so many others who find that hate soothes their soles and make them feel better about themselves, they seek an ever widening circle of people to hate. in particular they seek to hate and discriminate against those that they perceive as being moral, and living in accordance with the natural order. it is a big compliment (although perhaps not completely earned) that they single out jews and israel to be the targets of their venom.