I believe it started during or right after WWII, but I do not know the exact history. In context, it has all been part of the left’s desire to make the carrying of weapons verboten outside of very limited contexts. Many countries in the world do not even allow the police to take weapons home with them.
Perhaps someone else can say just when it started. Maybe it was right after the Civil war. I think, for a long time, no one questioned the right of officers to be armed anytime and nearly anywhere, but after WWII, that was gone. A great many in the elite in this country love the British model and wish for us to emulate them.