The Euro Chassis Cabs compete more with say the 5500-series trucks, do they not? With 19.5K GVWRs but a pickup cab, height and footprint (depending on frame length chosen).
I know some of the pickups available in the UK like the Hilux and L200 have useful 2,500lb payloads and fuel efficient 4-cyl diesels. The main advantage to the American pickups is in towing, where the engines since 2000 have offered tremendous power. Braking and frames are also evolving and with single rear wheel conventional pickups being able to seat 6, easy to park yet handle pin weights of more than 4,000lbs. That kind of payload is also handy for landscaping duties etc too, with or without use of a trailer.
For the OP, one word of warning re the new F150 regular cabs is their crash safety, though Ford may have re-added the structures that are present in the crew cabs. I hope so anyway. No doubt the HD payload package sounds like a must have option, strange they seem so hard to find on lots. As others said, likely due to a lack of education re these kinds of vehicles.