Because a corrupt group of gun profiteers are making billions selling military weapons to civilians. They are only able to stay in business because the NRA bought legislation protecting gun makers and sellers from liability for the destruction caused by the products they make and sell. 

They base the right to do this on a Constitutional Amendment written over two hundred years ago when "arms" were swords and muskets. And every free man was expected to serve in the army in time of war. I say we start drafting all owners of guns and sending them to Iraq and Afghanistan where they can put their guns to good use. We won't need to waste taxpayer funds buying them guns or ammo - just like the Constitution provides. The problem is that most of these weekend warriors are mentally ill cowards like this shooter.