I think it was more or less the best tool for the job (according to them) back when their tech lead chose it and now they've built all their tech stack around it. I remember a blog post or interview where he said they wouldn't necessarily use OCaml if starting again right now. I don't think he was referring to going the way of Haskell either.

Jane Street has big cult energy though you have to admit, if you've known anyone who worked there