That's simply not true. There are numerous android and iOS apps that support it. There are implementations for Vim and other editors besides Emacs. GitHub supports it, for example.
I have not found a satisfactory (to me) solution for note-taking in Markdown that is open-source, plain-text, and mobile-friendly with local (non-cloud) sync. (Except for maybe Logseq, which appears to be moving in the direction of using a database.)
There are several such solutions for Org-mode.