Is it though? Like for example, I often deal with Org-mode documents of several thousand lines of text and I honestly don't know any piece of software that can acceptably handle multi-thousand lines of markdown.
Emacs/Org-mode has tons of different ways to navigate and search through these large bodies - the outline nature of the structure is perfect for that - there's narrowing, collapsing/expanding, sparse-tree search, flexible sorting, indirect buffers, imenu, overlays and text properties that can render the text conditionally, etc.
I read HN and Reddit threads in Org-mode format¹; I browse my Jira board and tickets in Org-mode², I have Wiktionary lookup³ and Thesaurus⁴ - all in Org-mode.
¹ https://youtu.be/ud3Gmxg5UZg
² https://github.com/agzam/go-jira.el
³ https://github.com/agzam/wiktionary-bro.el
⁴ https://github.com/agzam/mw-thesaurus.el
Comparing Org-mode and Markdown and saying "Markdown is widely used [and thus it's better]" is wildly immature - popularity doesn't determine fitness for purpose - PHP is more widely used than Rust or Zig, but that doesn't make it "better" for systems programming.
I can agree, Markdown is adequate for relatively small documents like README files, but it's nowhere close to even try to compete with Org-mode in so many different aspects far beyond just the markdown format structure.