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...
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.