Another factor here is that until around 1980, police departments generally did not believe that serial killers existed. The many well-documented serial killer cases of the 1970s changed this thinking. Bill James's Popular Crime is a very interesting book that spends a lot of time discussing this.
The excellent Netflix series
Mindhunter is all about the FBI agents who established the first studies on serial killers and the psychology involved and quite interesting, people really don't seem to realize just how new most of our knowledge on how serial killers operate is.
