Why did Tarantino leave out the fact that Charles Manson was a white supremacist?

The movie doesn't spend a lot of it's runtime dealing with Manson's cult or get too deeply into what was going on there.

IIRC, Booth has a couple of run ins with them before the big confrontation at the end of the movie.

The focus of the movie is on the friendship between Booth and Dalton.

I would guess because he didn't want to do that much of a deep dive. Explaining their motives would've taken time away from the already long movie. Maybe he couldn't find the right spot for it? Plus if it's not fully explained, it adds to the intrigue of the story. It was just narrative choice, and I think a sound one. Clearly QT doesn't shy away from racism in his movies, and I don't think he tends to pander to anyone, let alone conservatives. I don't make it a point to follow all celebs' politics, but if anything he strikes me as more libertarian

Movie's not about him.