Difficult as in you are looking for Erlangers but cannot find them? I haven't seen many postings looking for Erlang programmers.
(I'm in the SF area, but a lot of the ads I see are for New York and parts of the Midwest.)
* it has to fill a need people have.
* people have to know about it.
* it has to have a mature implementation.
Age helps with point #2 and #3.
With regard to JavaScript, the web browser pretty much forces you to use it. So it doesn't really matter how good or bad it is, it will have adoption.