These kinds of guns have been around for decades and decades. Yet this was extremely rare in the 1950s, 1960s, 1970s, 1980s, and 1990s even up until Columbine at the end of that decade.
Then it's been way more frequent in the 2010s in particular.
The guns haven't changed. Something else has changed.
The 24/7 media obsession with these shooters is a part of it. Kids who grew up constantly hearing about shootings like Columbine are now getting old enough to start doing the shootings themselves (actually that was the case with the Sandy Hook shooting which was over 5 years ago, soon that will be the shooting the kids grew up obsessing over).