Coils can be made with high spring rates, my Expy EL has a payload of 1672lbs and GVWR of 7900lbs. It has coils.
It has more to do with the target customer. The majority of 1/2 ton truck buyers DO NOT tow with them, and of the ones that do tow, most of those are sea doos, boats, ATVs, or cargo trailers. Only a very small percentage of 1500 buyers are planning to tow a travel trailer.
The target customer wants a well appointed spacious interior, comfy ride, the ability to carry a bulky item or dirty sports gear home, and pull some toys on the weekend. In those respects Ram has hit their mark. Most people looking to tow an RV, are already looking at 2500 or 3500 series, not 1500. So Ram has taylored the 1500 to its target market. "Feel how smooth OUR 1/2 truck rides!!!"