If you folks are trying to say that LTs are tested to tougher standards than and ST there is no argument there. LTs are a more rubust tire because they need to be for their intended service.
If you are trying to say the more robust testing makes them better for TT service, that is simply opinion.
Those opinion is off set by 1.) The fact that TT and most of the other trailer industries find STs a sutable and economic tire choice. 2.) The fact that LTs are not widely used or advertized by their manufacturers for trailer service.
People can invent a thousand reason why this is true, again just more opinions.
My opinion is STs are used because they do what they are designed and tested to do.
It is also my opinion that those who, go off-road, drive fast, exposed their rigs to other extremes or simply feel they need them; would best be served by a tire designed for that purpose.