The more I think about this I believe the answer is simple.
The trailer industries, boat, horse, cargo, utility and RV wanted; 1.)A stiff walled tire to resist the side torque while turning a multi-axled trailer. 2.) a low rolling resistance straight running tread. 3.) A top speed of 65 mph. 4.)a reasonable life span. (5 years).
This would be a specialty tire (ST) designed and tested to meet just those 4 criteria. Without having to worry about traction, steering, road noise, high milage and a host of other condition expected of powered vehicles, an inexpensive tire could be made.
Given the fact that over 1,000,000 TT and 5ver tires are put on the road each year, and have been for at least the past three years, plus all the other trailer and RV useres, I would say they met their goal.
If you want a more robust tire for the your TT that is fine, but I never seen any data that shows the failure rate of STs are any worse than LTs or Ps.
How a person justifies his expeditures is of little interest to me. I have enough of my own biases.