Where do you live? This was also my experience moving to Spring, TX (a suburb of Houston), which I'm now leaving because of this type of attitude, among other things.

It's not just NextDoor, many areas are on average racist and insular. One time I was strolling my baby, and there was a group of well-dressed, but wrongly colored, people (men, women, and children!) walking down the street. A woman pulled over in her shiny car to ask me if I knew these people, and I walked on and ignored her question. When I looked back, she had pulled over and was on her cellphone.