The original reason for nickel plating was probably cosmetic only, but in later years they were sold primarily to people concerned about rust; for example, detectives who carried a gun in a shoulder holster under the arm and people who kept the gun on a boat found that blued steel rapidly turned into a mass of rust. That need has been met by stainless steel, and there were apparently not enough buyers who liked the shiny appearance to justify the expense of keeping the guns in the line.
Nickel plating those guns involved not only the expense of the plating setup but the special work on parts to be nickel plated to account for the tolerance differences in close-fitting parts like the sideplate and crane. When we sent parts to be nickel plated, we had to be careful to work down some edges, otherwise the plated parts would not fit in the plated frame without taking off some of the new plating.
Jim