It's bbcode formatting[en.wikipedia.org], not html.

All 3 are markup languages, and markdown causes problems with formatting stuff particularly math with * leading to the need to use the escape character "\", and often has to use brackets anyways for complicated functions like embedded text urls.

I don't see Valve changing it. They'd also have to maintain compatibility with both the old BBcode formatting and the Markup one. As there's a buttload of content already written in BBCode.

I've always wished the Discussions had a proper WYSIWYG editor, instead of having to write manually the BBCode tags as if I was web-developing in the late 90s