Here's what I don't get. You can make a thin walled stainless tank as long as it's shape is maintained by high internal pressure, at least when you need good structural strength. And that pressure would be like what, 4 atm? Which is great in the tanks but not so good for the large portion of the ship that houses people. Could you get away with balloon walls and low pressure if you had ring bulkheads (floors) every 2-1/2 M or so? Hmmm, just thinking.
My guess is 360 covering of pica and rotate during re-entry.
https://twitter.com/elonmusk/status/1071578086418788352
Going to a heavy metal is definitely counter-intuitive.The only thing I can think of is that even though it probably lacks the strength to weight ratio of carbon fiber the shift to metal construction must let them cut enough weight elsewhere to make it worth the switch.
Sea Dragon. 8mm thick steel sheeting. Sea Launch. Cheap and known. It makes sense. They could go back to 12m diameter to throw over 150mT+. Won't be 550mT though like ITS or Sea Dragon.