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.