I understand where the author is coming from and respect their contributions to Commonmark. But... There are tons of markup languages for prose that have well-defined specs. So, why did Markdown win? IMO, because it does not have a well-defined spec. It is highly tolerant of formatting errors, inconsistencies, etc. If an author makes a mistake when writing Markdown, you can always look at it in plain text. Whereas a…
Asciidoctor has a spec, reads pretty similarly to markdown, and is infinitely better IMO. And it (well, AsciiDoc) predated markdown! I think markdown won because it was specifically made with HTML output in mind, instead of arbitrary output (docbook, in the case of AsciiDoc, which is pretty much infinitely malleable).
There are in effect two different versions of AsciiDoc, because Asciidoctor people have appropriated the name while making their own changes to it and marking what they dislike as deprecated.
AsciiDoc cannot express all of DocBook, for example figures with multiple images.
While I despise Markdown, there isn't all that much to be a fanboy of. Just the syntax is overall saner.