Being the author of KeenWrite, a Markdown editor that uses YAML variables (for content and metadata), I'm biased, but here's my take:
1. Writing <p> tags for every paragraph gets tedious.
2. Markdown is succinct (**bold** vs. <strong>bold</strong>).
3. Content is easier to edit when not surrounded by angle brackets.
4. Content is more robust against markup gaffs (e.g., missing closing tags, tag typos).
5. HTML mixes structural markup with content, making design changes harder.
6. Markdown's simpler format is more portable to other output formats (PDF, ePub).
7. Markdown diffs are easier to review, and likely have fewer merge conflicts.
8. Non-technical contributors are more comfortable with Markdown.
9. Embedding metadata through meta tags is messier and harder to parse using raw HTML.
Here are a couple of blog posts written in Markdown and converted to HTML using my software:
* https://keenwrite.com/blog/2025/09/08/feature-matrix/
* https://keenwrite.com/blog/2025/10/15/creating-catchy-cover-...
The user manual is a set of Markdown documents converted to PDF (note how the words TeX and ConTeXt and LaTeX are typeset; try to do that with HTML):