You simply didnt hear about them but they happened...
https://en.m.wikipedia.org/wiki/List..._United_States
They simply became more common as semi auto guns became more common.. You dont see many mass shootings with bolt action rifles or 6 shot revolvers... I think maybe the only exception was the Texas university shooter who used a bolt action rifles but he locked him self at the top of a tower so it was hard to get at him but even still he had multiple semi auto pistols and a sawed off shot gun.. He was also a trained Marine Marksmen so it would be obvious for him to be capable with a bolt action..
24hr news coverage and the internet probably helps add to the mix as so many of these new shooters seem to want to copy cat the previous ones..