Original SGML was actually closer to markdown. It had various options to shorten and simplify the syntax, making it easy to write and edit by hand, while still having an unambiguous structure.
The verbose and explicit structure of xhtml makes it easier to process by tools, but more tedious for humans.
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.
Previously its popularity was somewhat similar to RST.
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.
And markdown tables are harder to write than HTML tables. However, they are generally easier to read. Unless multi line cell.
Especially for casual users of HTML.
“Always close your tags” is a simpler rule (and fewer rules, depending how you count) than “Close your tags, except possibly in situations A, B, C…”.