ORG's tightly integrated into emacs & it's ecosystem. It's fine but I don't think there is an equivalent to Sphinx for large cross referenced documents, with different outputs formats & support for translations (different languages).
Perhaps some of these things exist - I may be wrong, if they do I doubt the system is as mature or well supported as Sphinx's.
Jokes aside, org seems to have a more clear specification and is very easy so use. Why don't more people use it?
I think Github already renders README.org as good as README.md and READEM.rst.