Yes, exactly that. Org predates Markdown, however the Achilles heel is that it's tied to Emacs. I love Emacs, but I can't (ethically and pragmatically) force any editor/IDE on anyone. And as soon as a lone emacser commits an org file to a repo, it creates a disconnect.

Markdown (although it doesn't adhere to a strict standard) is ubiquitous, and support for Ledge notebooks degrades gracefully. If one doesn't have Ledge installed to run it, it can be assumed that their editor is able to render markdown. Even opened in a plain text editor, markdown is a lot easier on the eyes than org (IMHO).

UX matters.