You want a light weight and strong material because weight is a premium on a portable device. Once you identify weight and strength as the constraint there are very few materials that meet that criteria where weight&strength/cost is the controlling factor. Aluminum and Magnesium alloys, carbon fiber and few other materials are your only choices. Then it becomes all about the cost and availability.
The fact is that in this category you have 2-3 materials that work and it's a matter of function and cost, not a patentable design element. You can't put a design patent on a functional item, that means if you have to use a certain material to meet the requirements you can't put a design patent on that material.
Apple's behavior in this is atrocious. Like another poster, they are on my list with Sony and I'll never purchase one of their products willingly.