Paul, I'm not sure specifically about why 15" rims are normally the starting point for LR D tires. I did notice that the highest LR rating you can get with the 5 lug pattern is just over 2000 lbs. If you want more, you have to get the 6 lug pattern.
It may also have to do with the size of brakes required for heavier trailers. If you are pulling a 6000 lb loaded trailer requiring a higher load range, you will need bigger brakes which needs a bigger rim.
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