That's not really true. org-mode (2003) and markdown (2004) only have a year between them. Markdown was pushed more broadly (versus org-mode) because it was not tied to a specific implementation in a specific editor and was just for marking up text whereas org-mode was meant to tie in with more of emacs' other packages and capabilities.

If you weren't already using emacs, you likely wouldn't have known of org-mode in the late '00s. Markdown had already been incorporated into a lot of static blogging systems and forum comment systems by then with many independent implementations. Additionally, org-mode wasn't made a default element of emacs until 2006, two years after markdown was publicly documented.

All that said, I still prefer org-mode and use it for my own readmes outside of actual work where I stick to markdown.