Because (in the US at least), gay people started more openly coming out in the 1970s, with large increases in the 80s as well. So that's 30-40 years of gays becoming increasingly open.

Not to mention before that, there were people, even in the small town south, that everyone knew was gay but just didn't discuss in open terms.

Now it's true the cultural stereotype for "gay" was the Village People or Liberace or whatever, but a large number of people knew ordinary gay people as well.

Of course, even today, and even in a place as open as San Francisco, I suspect most everyone knows more gay people than they think they do. I know that's true, in fact. A lot of people just never talk about it.