Markdown has 30 flavors because it has over 30 different ecosystems using it. We have github flavored markdown, gitlab flavored markdown, MDN markdown etc. because github, gitlab and MDN are all using it. If you forced everyone to move to rst, then you'd see flavors cropping up soon too.
Even if rst has a generic extension mechanism, on github you'll spend a lot of time referencing commits, tags, users, pull requests and that kind of thing. So you end up having to know a domain-specific language anyway, whether it's formally presented as an extension mechanism to the language or as a dialect.
Even if rst has a generic extension mechanism, on github you'll spend a lot of time referencing commits, tags, users, pull requests and that kind of thing. So you end up having to know a domain-specific language anyway, whether it's formally presented as an extension mechanism to the language or as a dialect.