Not to knock Mermaid but the interesting thing about Markdown to my mind is that it’s still readable in raw format.

By which I mean it looks like a well annotated ASCII text file rather than a raw markup language in text.

Ditta looks like at least has the potential for that. If github had the ability render Ditaa it would be brilliant.

I think the main problem is that you still need the right tooling to draw it efficiently and Markdown doesn’t really require that. Eg editor plugins for reflow and drawing/resizing boxes.