The original spirit of Markdown was to use HTML elements (or custom elements if you like) for whatever is missing from Markdown. That's surprisingly versatile in hindsight, but the specification didn't fully anticipate what happens to Markdown contents inside such elements. Some implementations supported them, some didn't, some used the `markdown` pseudo-attribute, and so on. And it was even less clear how block syntaxes work inside HTML elements. (CommonMark defines a very lengthy list of rules for them [1].) Markdown could have been extensible... if it did have a sensible specification from beginning.

[1] https://spec.commonmark.org/0.31.2/#html-blocks