rST is nice for making complete documents, but for smaller things it just is not practical. For one-off things (like github comments) or smaller wikis, markdown is just more practical. rST has bigger reliance on a mildly smart editor than markdown has, just to handle indentation.

The upside of rST is that it's just more complete, it has more document elements available. Markdown is often written without thought to having a readable plain text document, but rST is often very readable in plain text too.