Why don't the manufacturers recommend LT tires??

All prejudice and opinion aside, there are design advantages to ST tires for trailers.

A couple of tire experts will come along and make the case for ST tires. If they are as terrible as some think, the ditches would be full of overturned travel trailers and fifth wheels. People who are willing to change out their original tires for LT's are the kind of folks who are more likely to protect their investment by avoiding curbs and maintaining proper air pressure.