The spread of military style weapons. In the 1960s, guns were actually more widely owned in the US but almost all of them were either revolvers, shotguns, or bolt action hunting rifles. It's certainly not impossible to carry out mass shootings with these types of weapons, but they've got a more limited rate of fire. Charles Whitman, whose 1966 shooting spree from atop a clock tower at the University of Texas is often considered the first mass shooting, killed "only" 17 people, using a combination of bolt action rifles.
Today's mass shooters have easy access to military style semi automatic weapons. A semi automatic weapon can fire bullets at incredible speeds. Gun control opponents will often make much of the fact that fully automatic weapons are all but illegal for civilians to own but that's almost a distinction without a difference. A shooter can fire some of these semi automatic weapons at rates which are virtually indistinguishable from full auto. Military fire doctrine also calls for soldiers to use semi automatic firing in most cases in order to conserve ammo and increase accuracy, thus meaning that there's often little difference between what civilians in a mass shooting might face an what soldiers on a battlefield might. Along with greater rates of fire, civilians have also been given access to larger magazines, which increases the efficiency of mass shooters. Every time a mass shooter has to stop and reload is a chance for people to flee or even try and stop him. But groups like the NRA have promoted large capacity magainzes which can hold massive amounts of ammo and which can be allow for a gun to be reloaded in seconds, thus increasing the time that a mass shooter can spend killing people. In addition, the rise of military style weapons has increased the lethality of mass shooters. During the decade that the assault weapons ban was in effect the number of fatalities from mass shootings dropped by almost a third. One of the reasons was that the light weight ammo of the AR 15 and similar weapons is especially deadly (that's why the military likes it). These rounds are less likely to go straight through a victim and more likely to pitch or tumble around increasing the likelihood of destroying crucial organs (if hit in the torso) or just carving up masses of blood vessels, thus letting the victim bleed out. That's a great feature when we're talking about US soldiers using it against our enemies but it's terrible when a mass shooter is using it against innocent victims.