Comment by marcinzm

Engineers like code and dislike fuzzy things with hidden edge cases. When Slack tried to switch from markdown to a UI engineers complained loudly. There's no reason to believe it'd have been any different had one of the fifty other formats that existed won out.

From a practical point of view, markdown you can diff, you can copy it around without edge cases, etc. There's a reason people use Latex rather than Word which is evident the first time you have to spend 30 minutes fighting Word's formatting edge cases.