Because to all the people she is living with on Earth for hundreds of years, she is a black woman. It doesn't matter that she is an alien, because she can't tell people that, and they can't recognize that just by looking at her. She has lived the life experiences of a black human woman multiple times over and has come to empathize and identify with them. She's walked the proverbial mile in another person's shoes, so to speak, and they share common cause now.
I liked the scene in "Django Unchained", where Chhristoph Waltz's character (Dr. Schultz) was "undercover" together with Django to buy Djangos wife from Mr. Candy. And they had to "play an act" undercover. And the reality of the situation hit him so hard that he started blasting, killing Mr. Candy, and blowing their cover.
That was a real good scene of someone being overwhelmed by the reality of the situation to start acting irrationally - in a way that was wrong and illogical for the situation, but very human and understandable to why he couldn't do anthing else.
I just don't think that kind of threshold was met by San Francisco in 2024. Especially for people that should know their history. It'd be way more difficult to witness, say, the Holocaust and doing nothing (see "killing Hitler in the past"). But homelessness, ICE, drug/poverty crisis just doesn't reach that level. All these characters have visited planets with WAY worse problems during their Starfleet time, active war zones even, and just witnessed the diaspora of the Romulan people.