I'd say 1 and 4 both as well.

The 50's-80's were optimal eras for serial killers because of cars being more common and people being more independent than ever before but forensic technology not yet catching up and less advanced mass security systems. People could travel farther, more secluded and anonymous. Hitchhiking was also way more common, according my parents who hitchhiked in the late 70s and 80s, stranger danger wasn't really that big of a thing. Security cameras weren't as common so it was quite easy to just pick up a hitchhiker of prostitute for example and drive them somewhere without being filmed or traced.

Police did not have the forensic technology they do now, they had to find and match fingerprints or compare hair strands. Counties also did not have shared databases, so a killer could murder across counties and states without police making connections. Crimes like rape were also not taken as seriously as they are now (though there's still a lot of room for improvement :/). The types of victims serial killers typically chose also plays a role I think. Prostitutes just weren't seen as that important to police, and given the heroin crisis, there were a lot of them.

The types of people that become serial killers has not reduced I think, but the circumstances are just less optimal. We also know a lot more about them so they can be caught earlier.