Pure fear of the unknown, combined with fear of getting less-than-stellar tracebacks & profiling because Elixir's only one of the VM languages.
Makes sense. For what it’s worth we’ve been really happy with traces, no issues there, and there are definitely profiling tools. Plus you can point Erlang’s Observer tool at a production BEAM node, it’s wildly useful!
NHS and Boston MBTA both run BEAM in production, and arguably lives are at stake there (mostly indirectly but still) ...