I think the reason you see more cube vans than step vans is that there are more made. My exposure to the commercial truck field tells me there are many more "hauling" applications for cube vans; the larger ones have standard dock height for easy loading and unloading with forklifts. Only industries requiring the driver to easily "step" into and out of the van (route drivers; laundry trucks) multiple times a day are interested in the lower deck height of a step van.
I was picking up some parts at my local commercial truck supplier for my Peterbilt recently. His lot was full of new and used cube vans of various heights and lengths; but only 2 step vans. On the way home I passed a Ford dealer, and way back in the raer of the lot were two brand new step vans. Couldn't help myself; I turned in to look. Pretty sweet, although a bit long for me (probably 18'), but talk about cube space utilization, WOW! White, diesel, all aluminum, twin doors, barn doors on the rear; what's not to like!
Just my opinion.