Used steel trailers are around as people get tired of the rust and upgrade. Not near as many aluminum used trailers out there as people buy them and keep them forever and they still look good.
Buying a steel 1200 dollar trailer when an aluminum trailer next to it for 3000 is penny wise and pound foolish. In a few years that 1200 dollar steel trailer will be worth its weight in scrap metal. And that 3000 aluminum trailer will still be worth 3000. Not to mention all the extra gas you'll be burning.
My all aluminum 2 horse gooseneck with nice carpeted 4 foot tack area and gooseneck sleeping area, weighs the same as my steel 2 horse bumper pull with lousy storage and no where to sleep. I weighed them both on truck scales to satisfy my curiosity.