Personally I think Markdown got _really_ popular not because it is easier to write but because it is easier to read.
It’s kind of a huge deal that I can give a Markdown file of plain text content to somebody non-technical and they aren’t overwhelmed by it in raw form.
HTML fails that same test.
Or because it was the default in GitHub with an ad hoc renderer.
People had already ditched writing HTML for years before Markdown came out.
People were just using other markup languages like rST.
Other attempts had already proven HTML to be a bad language for rough documentation. Someone then just needed to write a spec that was easy to implement and Markdown was that.