Just a minor note on this: Org-mode was created in 2003 (the syntax a bit earlier) and Markdown in 2004, so I think this was a case of parallel evolution, each unaware of the other, not intentional deviation.
Edit: And the use cases were different. Org was designed as a syntax for an outliner, and Markdown was designed as a syntax for prose. Both are plain text and optimized for readability in their pre-processed form, but maybe it's the fact that they are so similar that's remarkable.