OCaml is a great language, but IIRC the creators didn't talk much to the community, about where they were going, etc. Contrast with Clojure, Golang, Python...
So many people steer away and go bet on something else.
[1] http://anil.recoil.org/papers/2010-icfp-xen.pdf [2] http://caml.inria.fr/consortium/
The purpose of the language was research and teaching. I think it was never intended to be "battery included". The fact that so many people use it for real work and that so many libraries have been developed has a simple explanation: the difficulty to find a good language to have the job done. OCaml is a very good language.
I think it is easier to learn OCaml than haskell. Scala is more recent than OCaml and is more "battery included". I think that Scala will progressively replace OCaml and it a better fit for professional work.