In fact I am surprised it hasn't already been used by terrorists on western soil. I don't see how one can protect a VIP against a drone loaded with explosives flying quickly.
No one will be allowed to deploy automated anti-drone guns in populated civilian areas of Western countries. They would be tremendously expensive to install and maintain, would present their own security risks, and would endanger people if they ever fired at a wrong target or missed. Those systems will only ever be used to guard military facilities and maybe select sites in Washington DC.