The lack of non-docutils implementations is easily explained by rst being essentially impossible to parse; it's also somewhat annoying to write, in particular with the distinct lack of nested markup and the very baroque indentation rules - try writing nested lists without accidentally putting half of them in blockquotes. That being said sphinx-doc works very well even for complex projects and most documentation tools are rather inferior, much like static site generators it's a straightforward project to get to a "works for me" point.

rst (and sphinx) are also one of the few (open source) options which have some support for multiple modalities.