Haskell and OCaml try their best to not be popular. Elm is very beginner friendly, docs, installation, great libs for each purpose and is like 5 years old. OCaml has more than 20 years old and for a beginner to install everything and make a REST API is a pain. Why that happens?
>OCaml has more than 20 years old and for a beginner to install everything and make a REST API is a pain. Why that happens?
Nowadays things are quite neat with opam/dune/odig stuff. You can install packages, generate readable docs for them, just like in rust. For the rest api you could just take any existing framework, I think, e.g. [1] [2].
You should also consider that OCaml is a language, which for a long time was intended for a different kind of projects (yeah, webdev is not the only programming domain, shocking), so the direction was dictated by Coq and similar project devs rather then webdevs. Now facebook and other companies change the state of affairs.
