I don't think the lack of spec is holding Org back. Markdown came out in 2004; CommonMark came out in 2014. I think Markdown was already very popular by the time CommonMark came out.

The more obvious reasons are that Markdown was dead simple, unlike Org, and integrated into many popular products, unlike Org.

(Tangent: To be honest, I'm not sure why websites like Reddit opted for Markdown rather than text formatting buttons. It's a good thing, and I'm not complaining, but adding the standard rich text buttons (like the ones Reddit has now) seems like it would've been the more obvious move.)