[nuclearnerd]

It's all in the design. Aluminum isn't as much lighter than steel as you might think. It's only 30% the density, but it's (generally) only 50% as strong, so you can afford to use smaller pieces with steel. The ratio of strength to density is called "specific strength". For instance, a 0.75" x 0.75" x 0.065" (thin) wall tube is roughly as strong, a little less stiff, and only about 30% heavier than a 1" x 1" x 0.095" wall 6063 aluminum tube.

Welding (most grades of) aluminum takes away half of its strength, whereas mild steel stays relatively constant, so the steel welded frame would end up slightly stronger. Plus any hack with a mig welder could weld steel for you, compared to aluminum where you need a TIG welder and a decent amount of experience. Steel is a heck of a lot cheaper than aluminum too! These are good reasons to give (thin wall) steel a careful look.