Marketing is the main reason that the S&W J frame is produced in .357/.38Spl caliber. A small powerful "magnum" revolver has a strong appeal to the CCW holders. Even though the revolvers will shoot the .357 most owners will opt for the lighter .38Spl loads.
Smith was in a unique position producing the K and N frame .357 revolvers. The S&W guns were in high demand and selling well in spite of the difficulties some few were experiencing with the K frames. Colt was Smith's main competition at the time and the Python and Trooper models were admired for their mid size and ability to handle the heaviest .357 loads.
Smith decided to add a like size .357 to compete which resulted in the L frame. It was no coincidence that it was produced with a full lug resembling Colt's Python. The K frame 19 and 66models stayed in production long after the introduction of the L frame and are still sought after today. As ownership of S&W changed, corporate cost cutting and the demand for L frame spelled the end of the K frame .357s.