OP here -- first, this is not the same old thread about which tire is superior. This is a much narrower focus -- why would the tire makers rationally recommend ST rather than LT?
And I have gotten some very good answers. One, ST tires are cheaper to make and generate higher profit margins. Not sure if that is true. But if is, that's a rational reason for the companies' behavior. (Maybe not a nice reason, but rational!)
Another possible reason is that the sidewalls of STs are stiffer. I think that is true, based on what I have read. That would mean that they might be better suited to highway towing, leading to fewer product liability claims against the manufacturers. (I can tell you from personal experience that all reputable domestic companies are very interested in avoiding or minimizing litigation. That is not always true of no-name companies in foreign countries, who sometimes put out shoddy products and then disappear when the litigation hits.)
It is clear, though, that a sufficiently beefy LT tire can be a suitable trailer tire, based on the testimonials on this site.
