I think there are several mundane factors at play. Well, I think there are three.

(1) assume one in a million people are crazy enough to mow down a crowd. Right now, that means there's 320 people walking around these United States, waiting to blow. In 1995, there were 260. In 1975, there were 215. In 1955, there were 165. In 1900 there were 76. We're twice as likely now as we were 50 years ago through sheer dumb statistics.

(2) I remember getting a color television. It had four channels. There were three newspapers, the local (which didn't come out on Thursdays or Saturdays) and two state-wide. News was on at six and ten because the Mountain Time Zone is demographically irrelevant enough that nobody time delays it. In the time since I've learned to read we've gone from rotary phones to touch tone to answering machines to cellular to PCS to texting to LTE. The Internet has gone from a thing with acoustic couplers and daisy wheel printers that talked to a select few universities to this thing you get on your goddamn watch. I did my book reports with encyclopedias and inter-library loan. I couldn't find every bit of information everywhere ever with a properly-formed query. That sort of thing makes information ubiquitous, which means when you want to look up every school shooting there ever was, it's a few keystrokes away. 20 years ago it would have taken a couple dedicated weeks at a university library to even get a broad overview.

(3) It's a lot harder to die these days. No really. It is. We had any number of cars with cocktail skewer steering columns. I remember Lee Iacocca talking about what an economic drain airbags would be. Everybody smoked cigarettes. AIDS would kill you dead if the Russians didn't do it first. Shit - where I grew up I had classmates who died of plague. Not to mention, you know, war. The US went from the Spanish-American War to WWI to WWII to Korea to Vietnam and then a big fat nothing. 76,000 dead in Vietnam, something like 400 in the Gulf War 20 years later.

People didn't really fixate on Ed Gein until the Internet. Ancient serial killers didn't matter much. But since then we've sort of started this escalation of stupidity where people are trying for high scores. Anybody remember Kip Kinkel? Dude got himself his own Frontline special until Harris and Kliebold and they got their own Gus Van Sant movie. And now the powerless white kids with no war to go get glory in rack up their high scores against each other, crazy as fuck.