As he said in the presentation, it was because at cryogenic temperatures (which is the important part when you're building a liquid oxygen / methane tank) 301 s

As he said in the presentation, it was because at cryogenic temperatures (which is the important part when you're building a liquid oxygen / methane tank) 301 stainless is actually _stronger_ than Al / carbon fiber. It also handles heat much better, which allows you to have a much thinner heatshield. Both characteristics combine to give a lighter rocket than one built out of Al / carbon fiber. The price is just the cherry on top.