It seems like the majority of the worst serial killers were doing their crimes from the 70s and 80s. Infamous people like Ted Bundy, John Wayne Gacy, Son of Sam, Zodiac Killer, Jeffrey Dahmer, etc were all during that time frame. WHY???
There is a sizable difference between crimes committed by the serial killers in your list.
Son of Sam: killed people with a .44
Zodiac Killer: killed people with guns and knives
Bundy: pedophile, killed girls/women after/during raping them, had sex with their dead bodies, decapitated some of them
Gacy: pedophile, killed boys/young men after/during raping them, buried over 25 under his house and in his surrounding property
Dahmer: pedophile, killed boys/men after/during raping them, had sex with their dead bodies, dismembered their bodies and cannibalized them too
Son of Sam and the Zodiac Killer (minus the cryptic weird stuff) use of guns and knives is done everyday in Chicago - there are many active Son of Sams in Chicago. The mafia (any of them) has Son of Sams on their payroll.
The latter three are extreme cases, but they are not exclusive to the 70's or 80's. There has been serial killers from the dawn of mankind, but mass media increased dramatically in the 70's and 80's and those 3 just happened to get caught during that evolution. Though the gap is wide, there is an estimated 25-50 active serial killers in the U.S. right now, and there are many grisly unsolved murders throughout this enormous country.