u/Bodark43 and u/quiaudetvincet have both discussed possible answers here:
https://www.reddit.com/r/AskHistorians/comments/8mfblv/comment/dznl5go?context=3
https://www.reddit.com/r/AskHistorians/comments/8pda3s/why_does_no_one_know_how_to_make_greek_fire/