But why would they use Elm instead of Elixir as the language?
There is already at least a start at a Erlang Javascript parse transform: https://synrc.github.io/shen/
There is already at least a start at a Erlang Javascript parse transform: https://synrc.github.io/shen/
The same reason Rails would use CS instead of Ruby -- its a lot less work to leverage an existing front-end language than to build a rock solid compile-to-JS option for your existing backend language.