An interesting commonality among the three examples you listed: they're all designed for a world when computing was a more solitary activity than it is now.
For Lisp and Smalltalk, image-based development is a genuine hurdle in the modern world. It complicates many aspects of modern highly-collaborative software development, including version control and continuous integration. I think that the language's respective hacker ethoses might also be a challenge in light of how often people switch jobs nowadays. I haven't used Smalltalk for pay money, but my experience has been that initially getting settled into a large pre-existing Lisp project does take more brain effort than it seems to with a language like Java. (In the long run I'd rather inherit a Lisp project, but I also acknowledge that first impressions are important.)
And BeOS was not a multiuser OS and didn't seem to have a clear path to becoming one. I'm pretty sure the main thing preventing it from becoming as much of a cybersecurity disaster as Windows 95 was is the simple fact that there wasn't any honey in that pot.
Image-based development was never good. So we pushed it back again and again, so that we nowadays have a single image component we named "database" and most of the work is free from its problems.
This largely occurs because the only real spec of rST is the Python implementation. And tools written in other languages are loath to include a Python library just to support one document format. While writing a new compatible implementation is much more work given the larger scope than it is for Markdown.
So every language has a markdown implementation, but if you want to use rst in your Rust based static site generator or your PHP based CMS or whatever, it's a lot more work.
Every language has its down markdown - but they are not compatible with each other unless you stick to the very basic which doesn't do much. Which is why every implementation has extensions - it isn't hard to add extensions and there are a lot of obviously missing things from the basic.
Yet at the same time I'm able to write Markdown that parses and renders correctly in VS Code, Obsidian, and the Rust-based static site generator I use. Sure, Obsidian has incompatible features I could use like [[Wiki Links]] but I don't have to.
That is easy enough as long as you are only writing a simple one page readme type document. To be fair that is the majority of what people write. However if you write anything more complex you will run into issues.
Is there any rich text format which is well-supported across multiple languages? The closest I can think of is HTML, but even that has its fair number of issues...
I'm reminded of all of the alternate worlds that didn't quite come to fruition, but could have (Lisp, Smalltalk, BeOS, etc.).