The main reason for ST type trailer tires to gain more pressure than the LT tires, is that they are forced to support more load relative to their size & inflation than LT tires are.
If you ever look at the Load & Inflation tables and find an LT and an ST type tire of identical physical dimensions you will see that the ST tire is “rated” to carry more load than the LT type tire is. The basic theory behind that increased capacity is that ST tires will be traveling slower because people should not be traveling as fast when towing as when just driving the car or truck. When ST tires were introduced they were limited to 65 MPH MAX in an effort to offset the damaging effects of higher loading than seen in LT tires.
Tire load capacity is basically a function of volume and pressure as seen here Load = K x (Air Volume x Air Pressure) with different type tires having different “K” factor. Tires in LT applications are required to support lower load as a percent of their volume and pressure so they do not have to “work”as hard so they do not generate as much heat.
More heat means a greater increase in pressure. (Approx 2% pressure increase for each increase in temperature of 10F)
Please note that the actual load calculation is much more complicated as the response to air pressure is not linear and different aspect tires i.e. 75 series vs 85 series etc have some different factors that are applied to the actual calculation.