All of the cultural points and access to guns points Brisket has made are true, but I would say they created the tinder box of a problem. What really set that tinder box ablaze the last 10-15 years was the Internet, social media, chat rooms etc.
People can get radicalized so easily online now. Some people get radicalized into hot peppers, some it's fantasy football, some it's van life and over landing, and then some it's the mass shooter.
Whether is Discord, or some dark corner of 4chan or IRC rooms, there are now people that talk each other into commiting these things and trying to out do the previous mass shooter, etc.
In years past, those people may be crazy loners who hate the world, but there was no community pushing them to actually commit the acts.
Now the crazy angry loners connect with other crazy angry loners and normalize the idea of mass violence with each other to the point where some of them go out and do it.