concur - evolution. old trucks mimicked old cars - picture a 37 chevy's fenders, for example... they were separate from the main body. Truck beds evolved the same way, just a little more slowly and the fleetside was born in the form of the 55 cameo. BUT, I'd assume for nostalgia reasons and marketing, the stepside was retained as an option because back in the day - many may have preferred the more "traditional" look of steppers. Also consider the actual function of the "step" was to allow loading/unloading from the side (not that you couldn't do it with a fleetside...) and working parties were perhaps larger back then (think, farm) and maybe it would allow for multiple folks to load simultaneously (left/right/rear).
Ok, enough bloviating. I'm just guessin here....
