I think about this often as an org mode user who uses it exclusively for journaling with none of the GTD features. Org mode was released before markdown by over a year, but never saw the uptake like markdown did, despite being a more featureful syntax. I think that's because org mode was originally a GTD framework for emacs, and the syntax of org files was incidental to doing GTD in plaintext. It didn't get popularized as an alternative to other markup languages until long after markdown became popular.

I don't really know. I wasn't around back then to watch it unfold. But I still much prefer org mode due to better emacs support and (IMO) more intuitive syntax for things.

For some time, I used org-mode for almost all type of note taking. I love the structure being functional without HTML rendering. However, it's pretty much locked to emacs and so the portability is very poor. I need most of my notes to be at least readable on my mobile devices.

When I discovered Obsidian, I decided to fully switch to Markdown. It's very nice that it supports vi bindings.