As a few others have mentioned here… it’s because they were black.

I’ve personally witnessed a racist try to explain to an South Asian friend of mine that he was “OK” because there’s black people and everyone else. I was flabbergasted because this wasn’t some nazi skinhead or something, but instead was an older white gentleman.

All racism is bad. But there’s racism, and then there’s racism in the USA towards black people, which unless you’ve witnessed it, it’s hard to understand how it is a whole other level of racism.