That's because early revolvers were not otherwise drop-safe. Any live round under the hammer would be fired if the hammer was struck. NAA's mini-revolver has a very elegant solution to this problem; I'm surprised I've not heard of it elsewhere (they allow the cylinder to be parked halfway between rounds).
Early Colts and Remington also were made with a safety pin or cutout on the rear or the cylinders between the percussion cap nipples.
The Walker had one pin; the later Colts one between each chamber.
I designed a custom hammer screw for my vintage Colt .44-40 SA that acts as a positive safety and works just dandy but during the Klintoon years gunmakers and custom part designs were targeted by the government.
I prefer to have all six chambers loaded.
Many very early Colt and S&W DA revolvers had internal safeties.