When and why did people stop carrying guns in the 1900s?

Answer 1

If you are asking about 'open carry' holsters that are always visible, then I would say that the practice phased out gradually as towns, counties, and cities developed better police departments. Carrying a visible firearm is intended to be a deterrent to attack and if you have law enforcement walking the sidewalks they offer that deterrent image.

But concealed carry never went away. The 'pocket gun' was very popular from the late 1800's through the 1960's when lawmakers passed gun control laws of questionable legality. In the last dozen years some of those laws have been overturned, some strengthened, but concealed carry licenses have been issued at ever increasing record levels for a half-dozen years.

It always gets to the point of when seconds count, the police are minutes away.