Well for one we redefined mass shooting a few years back which absolutely spiked the numbers because now any robbery gone wrong or gang turf war gets classified in as a MS.
So when people see 300 MS they think 300 columbines or 300 Aurora theatre shootings. Most of those are much smaller and have nothing to do with the lone wolf shooter that people think of when the words “mass shooting” come up.
The other reason is cultural significance. There’s an increased likelihood of a shooting in a place where a previous mass shooting has occurred. The media spending weeks covering the incidents while plastering people’s likeness all over the place shows the disenfranchised extremist that they too can become infamous in a blaze of glory which leads to increased incidents of mass shooting as well.
Lastly, I think there’s a general air of “there’s no hope and I have no purpose” among people today that previous generations never had to deal with in the same manner. Add that on top of the previously mentioned things and it multiplies.
So basically media coverage and the sociopolitical divide becoming more of a chasm. Access to firearms hasn’t changed much in that time and firearms technology had semi automatic weapons invented for much longer than that.