I am sure it was a slow change that we hardly noticed. I think a couple things contributed to this mentality (there are plenty of others): 1) The tragic death of Adam Walsh became national news and spawned his father's advocacy for victims of crime. That spawned "America's Most Wanted"; a show that convinced viewers that each person wearing a sleeveless undershirt was a criminal. Faces of missing people appeared on milk cartons. No one wanted their child to be next. 2) Cable TV news needs to fill up 24 hours a day, so it breathes fear into us with each story so we don't dare change the channel.
Or not...