This is a pretty solid list. It was just a different world in, say, the 70s and 80s when serial killers seemed to be more common. Mass shootings are the in vogue crime for the nuts who might have been serial killers 50 years ago. They know they won’t escape detection for long with individual murders, so if they’re going to be caught or killed by police at some point, they try for the “many victims in one go” approach. And that’s not to say there no mass shootings in the old days (U of Texas clock tower shooter, for instance), but the above list has to be at least partly a factor in why people choose mass shootings instead of something more secretive and drawn out.