Why do some families disown their relatives for interracial relationships/marriage?

I had a friend that had a relative being disowned because he is engaged to a black woman. If you don’t want to talk to him and her family says if she keeps talking to him, they will disown her for supporting their engagement and they don’t feel comfortable with it a all. her aunt and uncle do not want to have half black grandbabies. That’s awful. My neighbor said he and his wife no longer speak to their oldest daughter because she’s married to a Hispanic man. That is so stupid! I don’t really talk to them because they’re racist and very brutal.