There are other solutions which are as easy to read/write (if not more) and which are defined much better… Asciidoc comes to mind.

"Better specified" doesn't mean better. A bunch of features doesn't mean better.

Markdown is popular because it is simple. I have never had to teach anyone Markdown. They look at the file and immediately see what's going on and can copy the style.

Occasionally someone may have to look up something like table syntax, but no one in my career or personal life has ever asked me to explain Markdown to them, which is rare for a technology that is so widely used.

This has not been my experience with other markup languages.