The problem in practice is that quickly one merges into the other. You start with a markdown readme, then you have markdown documentation for a small project. But then one day you need full documentation for your project with cross links, translations, accessibility. With Markdown you end up bolting these things on and each flavor does it a bit differently.

Perhaps some of the blame can be laid with the poor UX of technically superior systems. restructuredtext (apart from the terrible name) built with Spinx can do impressive things but becomes a huge pain to configure. All the XML-based tools like DocBook are very complete but try to get started actually building something - apart from having to author them in XML (which is already a kind of punishment), then you have to figure out XSLT stylesheets, 2000s-era design Java tools for processing them. And just look at the DocBook landing page! AsciiDoc has improved their onboarding recently but does have the issue of feeling like a markdown-ish alternative that's just a bit different for no clear reason.