Why?
Because there's no reason anyone would need to own a silencer, machine gun, or sawed off. Isn't a hand gun enough for defense? I mean, unless the muggers are driving around in tanks, this is totally unnecessary.
Silencers in particular are only useful for crime or government stuff. If you're defending yourself, you want the loud noise of a gun in order to get people to call the cops or come and help or whatever.