I’ve requested Markdown in the past, the answer was always the challenge of making the formatting look the same no matter what platform you’re on: Web, iOS, Android, Mac, Windows, etc. Short answer: Last I heard, they have the system working, so they don’t want to change it up (if it’s not broken, don’t try and fix it).
That’s not to say that you couldn’t implement Markdown-style syntax; other editors out there have done just that.