My guess would be that the people who haul horse trailers want to use their beds more when they are empty, than people who haul 5th wheel RV trailers. The gooseneck has a much smaller footprint in the bed of the truck. Also due to the design, a gooseneck trailer will be heavier than a comparable RV trailer.
Gooseneck trailers are generally stronger than 5th wheel RV trailers, but that's due to how the trailers are built, not because the gooseneck is an inherently stronger design. IMO RV trailer builders tend to be more cost conscious than horse trailer builders because they are adding many more amenities than most horse trailer builders.
Anyway, that's what I think. :)