It's a trade off between functionality and user friendliness. You can currently do more with Haskell/OCaml/F# but Elm definitely has the edge when it comes to user friendliness. If that bothers you, just think of Elm as a useful tool in expanding the pool of functional programmers.
As someone who thinks OCaml rocks (but hasn't tried js_of_ocaml), it's not terribly well marketed (on top of suffering from not being new-and-shiny). Also, making and distributing a single-page OCaml-to-JS app means learning about its build system(s) (the nicest thing to say about it is that it's not best part of the ecosystem), OPAM, etc. It's not really sold as a one-stop solution for new programmers.
The other thing is that Elm comes with the "Elm architecture". A OCaml GUI app backed by the react FRP library comes with "here is an FRP library, figure out the best architecture yourself" architecture. There are other open questions such as "how can I combine this with the hundreds of thousand JS packages on NPM" and "can I use it with Webpack/Typescript/other trendy JS techs".
That said, the possibility of compiling pure-OCaml libraries to JS is quite interesting.
