HTML is internally consistent. The atom of HTML is the tag. It makes it easier to remember and to learn new ones. Markdown is composed of arbitary punctuation which is harder to remember.

For making something bold, there's low friction in wrapping it in asterisks compared to the HTML version. It's fast, easy and works.

For making a link, the friction is higher because you have to remember the link/text order, and the link/text punctuation. There's a what, one in four chance of getting it right if you haven't memorised it? It always trips me up and I have to go and look it up.

Whereas a link in HTML is pretty much the same format as everything else in HTML.

I think Markdown excels for short or limited things that consist of headings, lists and inline formatting, like blog-posts and comments and ReadMes, but I find it doesn't scale well.