> Its lack of flexibility is an important feature, too (no JS, no styles, and only one way to make text bold)
HTML has two ways to make things bold: b and strong. But only one way to make a toplevel heading, h1.
Markdown has one way to make things bold: ** **, but two ways to make a toplevel heading: # or ======
Other users have commented that Markdown in the wild is now little more than an alternative syntax for HTML, with scripts and other tags freely embeddable.
Moreover, HTML is basically HTML: we used to have problems with browser compatibility, but most of those are in the land of CSS and Javascript now. Whereas Markdown can be found in different flavors. Aside from the burden on the author to know what flavor they are writing, it limits the reusability of documents. Consider a git repo that has a Github-flavored README.md file written for github, but it is also shown as part of the package documentation in a language specific library repository that shows the same README.md file using a simpler flavor of Markdown. The basic tags probably all work fine, but the broken CI badges will perhaps make it frustrating to read.
HTML and Markdown both began with the best of intents as declarative document markup schemes. But because it is much harder to design a declarative programmer interface than an imperative one, both have accreted junk.