It's significantly harder to learn than most other languages du jour so people lose interest. This limits its popularity. People who already know how to program fail at picking up OCaml because the syntax is too unusual and they're not used to programming in a functional style.

Do any companies use OCaml?

OCaml industrial users include major software companies such as Microsoft or Citrix, financial companies such Jane Street Capital or SimCorp and critical software companies such as CEA or Dassault System. Industrial users of OCaml have documented the reasons they use OCaml and how they use it.

Is learning OCaml worth it?

As multiple forum posts attest, OCaml isn't easy to learn. However, it learning it can be worthwhile even if you don't get into Jane Street. ... It's not just Jane Street that uses OCaml, and it's not just Jane Street's developers who can code in the language.