Although I am not a mechanical engineer< they should know this by heart> there is a lot of things to determine what they want to build it out of.
Depending on the alloy, normal structural shape aluminum is about 1/2 the weight but only about 1/3 the strength of steel, in most measurements ie, load yield, a bunch of other stuff I really dont remeber,understand etc.... but pound for pound certain alloys of aluminum can easily outdo common steels due to a term called "section modulus". This is why most aluminum handlebars are usually a tad bit lighter yet a lot more resistance to bending than steel ones, they use a thicker walled tubing that they make the bars out of. Also the same reason why Honda frames are noted to be "stiffer feeling" then conventionl frames yet I believe a bit lighter due to frame design .
You also have to take in consideration of how the plate is bolted on. The material has to be thick enough that the bolts dont just rip through.
Aluminum has an excellent compromise between cost,ease of manufacturing, light weight and strength to fill what the engineers idea of the perfect skid plate should be that can be easily sold.For some reason a lot of people aren't to keen on plastic ."hey look at this peiceof crap costed me 150$ and its made out of plastic! Whats going on dont anybody make anything out of metal anymore?!?!?"
If you where to make a steel skid plate the same dimensions of aluminum without any wieght concern I am sure it would be stronger but would it be stronger than the frame you are bolting it too? I am not sure but if you landed on a sharp rock that could easily bend your frame rails, crack your engine case or if you had an aluminum skid plate bend that up, but if you had a steel indestructable plate wouldn't all that force just be transfer to the mounting location crushing/bending the frame rails there instead?