In a piece of the script from Spider-Man: No Way Home, it explains how Wong becomes Sorcerer Supreme, since Doctor Strange blipped. However, I don't think it is explained why Doctor Strange would have become Sorcerer Supreme after The Ancient One dies; he was only a student for (around) a year, when other sorcerers like Wong and Mordo have been sorcerers for (perhaps?) years...
DR. STRANGE: “Higher duties?!”
WONG: The Sorcerer Supreme has higher duties, yes.
PETER (to Dr. Strange): Wait, I thought you were the Sorcerer Supreme--
DR. STRANGE: No, he got it on a technicality ‘cause I blipped for five years.
PETER: Oh. (to Wong) Well, congratulations.