Why do many horse trailer manufacturers use mostly steel or aluminium or a combinat

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We have a Featherlite 9440, year model 2000. This eighteen year old trailer shows no signs of age, despite tens of thousands of miles hauling horses. We used to pull steel trailers that were rust buckets in less than ten years.
Article by Double D trailers, a manufacturer of steel trailers. Thanks, but I'll stick with all aluminum.