> Markdown is that it just doesn't do everything LaTeX does.

That's not the purpose of Markdown. Markdown is a quick language to write HTML formatted content,nothing more.It's supports a limited set of tags. It's popular because it's simple ,unlike LaTeX. It's popular because it's easy to write a Markdown parser. Simple tools are always popular.There is nothing more to it. A rant against Markdown is unnecessary.It was designed for a single,limited purpose.