Daring Fireball: Yours Truly on The Vergecast: ‘# the Epic Story of Markdown’:

But the biggest reason for Markdown’s continuing success isn’t Markdown itself. It’s the triumph of plain text files, both for system configuration and for the interchange of human-readable (and thus, LLM-readable) prose.

This. This is why Markdown is so successful. As expressive as HTML is, it’s not the first format I turned to if I want something I could easily read in a text editor. The ability to easily read Markdown files as plain text is one of its killer features.