It wasn't made clear in the last arc that neither of the Hashira had ever gone Super Saiyan before. That's important information for the audience to have.

Quote Originally Posted by neflight86 View Post
So is the consensus that the mark (totally forgot about it) comes during/surviving a do-or-die situation? Funny it spreads like a disease of swordsmanship.
Ancient book: "The mark will appear on one person. Then on the people around that person."
Lady: "The mark first appeared on Tanjiro, then on the Hashira that were with him."
Every Hashira: "So...we're all going to hang out with Tanjiro...right?"