But nobody ever gets away with a mass shooting.
Which means that, by definition, deterring mass shootings is difficult.
So, we need to prevent them. But the methods proposed for preventing such unusual and cloudy crimes -- putting a brave marksman cop in every school, taking away 300 million guns, locking up the weird, arming kindergarten teachers, etc. -- are not terribly confidence inspiring. Since nobody knows for sure what to do, this makes mass shootings ideal for arguing over. Everybody gets to expound on their favorite prejudice -- stomp on rednecks, hire gun lovers to sit in schools, or whatever -- without much connection to what will happen next.