The communities for languages like Elm are definitely smaller, but they're also "passion languages", if that makes sense. Nobody is learning elm because they've been or feel forced to (like you might with React, eg) or because they needed to maintain a legacy codebase at an old job or something. They're learning it because they're interested/like it, etc. So when it comes time to hire you have a small(er) pool of candidates than a react position might draw but the candidates are going to be people who've played around with Elm and enjoyed it and
want to get a job writing Elm.
I'm kinda surprised more companies don't take the risk - its not like these are bad languages, either. There's still a huge chunk people (in terms of absolute number, not portion of total devs of course) out there playing around with languages like Elm or Haskell or a Lisp/Scheme, or OcaML, F#, etc. who'd be super excited to use those languages professionally.