I can only tell you why 4xGuard does not use Aluminum. While it is lightweight and rusts transparently, it can't take full vehicle weight without undue thickness (costly, decreases clearance, adds the weight you're trying to save by using Al) and it is "sticky" -- by which I mean that if you hit a rock with it Aluminum will want to grab and hold the stone instead of sliding across it.

So I wouldn't recommend it personally.