It's intersectional. It's partially white ignorance, most whites didn't see the shit Black women go through; the voices of white women were elevated because of their whiteness so their own concerns came first in the public discourse; Black women were rationally reluctant to raise their own voices in the face of white terrorism; Blackness has been coded as masculine and male by white society to better superexploit their physical labor; increasing the population of Black labor was no longer seen as desirable by white society after the end of slavery, Black women were doubly undesirable for both being Black and giving birth to Black babies; white women believed they could easier acheive their own liberation by focusing only on themselves and excluding Black women.

Angela Davis keeps mentioning "All the Women Are White, All the Blacks Are Men, But Some of Us Are Brave" so I probably should read that at some point.

Basically it was playing on white supremacist ideas like "You gave black men the right to vote before white women??".

Among other things, it's respectability politics. Many worried that adding an additional radical notion -- Black Suffrage -- On top of their already radical position would push it too far past the window of achievable politics.

It's the same justofication sometimes given for why the LGB movement excludes T

It's fucking wild, isn't it? But racism goes so, so deep. Some people believe in it like they believe in the sun.

White supremacy ideology puts great value on white femininity, and often portrays white women as being under threat from men of color, and people of color in general. Lack of an intersectional perspective leads white feminists to parrot the tropes of white supremacy, with the language of feminism.