I think its primarily because that is the most common formatting in every editor now? I could be wrong. Markdown has become the standard for README files for over a decade now.
Winning a popularity contest doesn’t mean it’s good. That is the worst part of about these things as they just generate the most common denominator type code/tooling while also repeating anti-patterns/mistakes like the bash vs. sh-session/console issue I pointed out. Garbage in has been so much garbage out unfortunately.
Never said it was good, just making an observation that Markdown is most likely to be available to render OOTB in more editors. I don't think Markdown is bad necessarily either. It's "good enough" for simple document.
Documentation isn’t as simple document. There are tons of rich elements missing from the spec specifically for documentation… which is why so many resort to adopting one of the many incompatible forks of Markdown to try to get features that were missing (but are a part of reStructuredText & AsciiDoc for instance). That has a real tradeoff since these forks are not going to be compatible & they aren’t going to be as well defined as the specs for these richer lightweight markup syntax choices.
