The problem with reStructuredText at least is, that there seems to be only one canonical parser, that defines the format. Markup formats in my opinion need to be defined in terms of a proper grammar, so that we can easily adapt that grammar in any programming language to build a parser and have support for that format in another language. The Org format in Emacs also suffered from this, but now there is an effort to make a grammar for it, I believe.
That said, I have used reStructuredText for writing a technical master thesis, and for that it worked wonderfully. If you buy into the ecosystem or use Pandoc to convert to LaTeX/TeX, and build a PDF or whatever you need, it will work well. But if you want to use it as a basis for HTML pages from other languages, which don't have a parser for reStructuredText, then you are in for trouble.
> The problem with reStructuredText at least is, that there seems to be only one canonical parser, that defines the format.
The same is true of Markdown (the canonical parser being John Gruber's at https://daringfireball.net/projects/markdown/) but that didn't stop third parties from extending it in their own implementations. For example, canonical Markdown doesn't support tables at all, but GitHub added custom markup for tables to their parser ("GitHub-flavored Markdown") and it became a de facto standard.
Oh, I am surprised the original didn't include tables. Guess it has been too long, since I looked at his page.
reStructuredText of course has support for tables[1].
I think writing a grammar for reStructuredText is a larger effort of course than writing one for Markdown, which is not to be underestimated.
Though extending reStructuredText is way less necessary, because it already includes a concept for custom "directives" (iirc that is what they are called). For example I have once made a custom directive to link to other documents, so that I have a local "wiki" like structure made out of files and folders. In Markdown no specific syntax exists for things to be implemented as extensions. Customization authors need to include special things in their parser instead, or come up with an extensibility concept themselves.
I think the point you are trying to make is, that there seems to be something else making the difference in adoption of the formats. Do I interpret your comment correctly?
That said, I have used reStructuredText for writing a technical master thesis, and for that it worked wonderfully. If you buy into the ecosystem or use Pandoc to convert to LaTeX/TeX, and build a PDF or whatever you need, it will work well. But if you want to use it as a basis for HTML pages from other languages, which don't have a parser for reStructuredText, then you are in for trouble.