It's not obvious at all. The music industry has been using the term "master" for ages: master bus, master copy, mastering engineer. (It's worth noting this is the exact etymology of the git term, too, as opposed to master in the context of master and slave.) The Black community has overall had zero problem with this, and it hasn't stopped many Black musicians and artists from being successful. The music industry also had none of the "we have to rename the master bus" nonsense that came from the tech industry, despite, or more likely because, a significantly larger proportion of BIPOC and LGBT minorities being involved in music.
I don't care about the master vs main name in the abstract. Either branch name is a fine choice. It just completely doesn't matter and pretending that it has any positive impact or meaningful change on the racism and discrimination faced by Black people in America is insulting. It's purely driven by self-indulgent white people who don't want to make material changes to their own extremely comfortable lives while pretending they're fighting the good fight.