I believe the biggest reason 19.5" wheels are not sold on SRW trucks is vehicle comfort followed by reduced speed ratings. The majority of SRW owners use the pickup as a secondary or primary vehicle unloaded which puts too many limitations on the owners that haul infrequently. Those that need more weight carrying still have the option of going to a DRW with LT tires that do not hamper ride or restrict speed like the commercial tires.
My front Dana 60 axle is typically loaded 4500-5000 lbs and my rear Sterling 10.5" axle is typically loaded 7000-8000 lbs. I am a good example of someone that benefits from a 19.5" SRW or LT DRW pickup. You could argue that I should be driving a dually, but show me a dually that is under 20' long in my cab/bed configuration that is not a custom build...
I do realize there are limits to my configuration and we plan to up size truck, camper and trailer starting in the near future. As I upgraded in the past, this will happen with the weakest/smallest component getting replaced and subsequent upgrades happening as funds allow. Luck does play a factor in the research or method I am following.