I don't think anyone here, or anywhere for that matter has the answer to that question. Maybe part of it is a society that offers less and less hope to young people. The seventies and eighties were the last decades in which children could expect to become more successful than their parents. Maybe there is an increase in the sort of nihilistic worldview that these shooters have, spreading like a virus in the dark corners of the internet.
Maybe the Columbine shooting kicked the whole thing off? That event is etched in the memory of this country, perhaps the notoriety of it began a spree of copycat shooters that continues today. Maybe our culture of glorifying guns and violence as a solution to whatever troubles us is partly to blame. It's almost poetic that in an era of unending pointless wars abroad, we are plagued with unending pointless killings at home.
These are all ideas worth considering, but offer no easy solutions. What we do know is that America is unique in this regard, and our culture around the weapons used in these acts is also unique. Until we can figure out the root cause of the problem, it doesn't seem irrational to me to address the one element of the crisis that we do have control over- the ubiquity and easy access to firearms.