I suspect on reason is that it is very very very hard to find a 15 inch LT tire with a load range of D or E. the vast majority are LRC. So that means many trailers need 16 inch wheels to use LT tires with an adequate load rating.
So, that means cost of tires along with cost of wheels and in a fair number of cases different axle placement and spring placement to give enough room for 16 inch tires.
On my trailer the only way I could switch to LT tires is to also go to a 16" rim,
Now for trailers already equipped with 16 inch rims, another story.
