I'm sure a mass shooter would have happily used a suppressor by now if you deem them so effective in aiding a mass shooting and since mass shooters are going to break the law anyways. I bet you won't find a single mass shooting case where terrorists or domestic terrorists used a suppressor in any single mass shooting. Again, fear mongering without knowing the facts behind how suppressors work despite me clearly explaining it out that to make them even effective you have to use special and far less powerful ammo which is not ammo a mass shooter would want to use to cause mass carnage. 

Which leads me to another point; basic firearm silencers can be constructed from materials found in two aisles of a hardware store for less than $20. If would-be criminals were so inclined, in a few hours time they could fashion a firearm suppressor that performs on par with commercially manufactured suppressors yet there is no single mass shooting you can point out where one was used!