My parents watch Criminal Minds and similar shows all the time, two friends of mine bought a book about a serial killer and my Twitter timeline is full of people talking about a new true crime podcast. It's just my bubble or is it a broad cultural phenomenon?
Matlock and Columbo didn't deal as much with serial killer stuff. They were still mostly about mystery solving. I think Law and Order would be the earliest example for that. I also think that saturation of slasher movies through the 80s led to a going darker and grittier or 'classy and psychological' version of those killers became the replacements when inexplicable slasher villains went stale. If you want your scary killers to be more deep and real you look into real killers and then you just start selling the real stories.