Just recently there have been a few FS bikes popping up that are made from steel - Superco has DH bike (their connection with Brooklyn would explain why it's steel) and K-9 industries are about to launch a DH bike in steel. And o course there is Brooklyn with their esoteric (and exotic) Racelink.
Now time was when steel fell out of favour because it couldn't be made light enough and stiff enough to compete with aluminium. By the time you got to a reasonable weight reasonable stiffness frame, the tubes were so think they were prone to crushing and denting. The original Sunn Radicals were I think made of steel, but that was when the weight of a DH bike was less of a hot topic.
Now, the Superco DH bike is being billed as weighing 37lbs, which is pretty remarkable. The advent of new forms of steel alloys like 853 and 953 have heralded a new age of steel hardtail frames but not FS frames.
So, why not make FS frames out of steel? Is it just that everyone has tooled up for aluminium; perhaps there's just not any advantage to be had; do they still have a weight penalty/stiffness penalty (depending on how you use it) compared to aluminium?