There are a lot of special-purpose markup languages, like Markdown and AsciiDoc. They involve toolchains written in other languages to convert them to HTML. If you want to add features, you have to hack the toolchain.
For blog posts and simple websites, that's fine. But sometimes I want to build something nontrivial -- as an academic, the first thing that comes to mind is an academic paper, where I might want to have special markup for theorems, proofs, examples, figures, and tables, and have ways to automatically cross-reference them, generate tables of contents, embed automatically formatted bibliographies, and so on. Or generate figures from code (like TiKZ), or embed data analysis code right in the source and embed the results (via knitr or Sweave).
With pandoc, bookdown, and knitr, you can get pretty close to this. But what made LaTeX so powerful is that it is programmable in LaTeX. It can be extended: you can define new types of environments (example problems! homework exercises! every example shows equivalent source code in three different languages! category diagrams generated automatically! musical notation is typeset!), you can make new commands to automate drudgery (typesetting chemical equations! building complicated equations!), and you can do it all with at least some basic separation between content and presentation style. Converting the larger LaTeX documents I've made to Markdown or Org would be basically impossible without writing a bunch of scripts to extend the Markdown renderer and hack everything together. There's no equivalent of just writing a LaTeX package.
I'm not aware of another document preparation system that comes close to LaTeX. Org mode is probably the closest, since it has tools to embed code blocks in other languages and include their output, but after a week of fighting with conversions between markup languages I can really see the appeal of LaTeX's uniform syntax and built-in programmability.
The other promising option seems to be Pollen[0], a Racket-based programmable system. But it seems more like TeX than LaTeX: it provides the very basic tools to build a programmable document system, but not the higher-level conveniences (like cross-referencing commands and standard sectioning and environment commands) you would need to build yourself. Maybe someday...