Elixir is not a sandboxed language, so you can't just accept arbitrary Elixir programs from users and execute them inside of your application without security concerns. Lua, on the other hand, can be executed in a sandboxed fashion, limiting and constraining the reach of user programs.
Check out Anthony Accomazzo's post about Mini-Elixir, which does a great job breaking this down much further https://blog.sequinstream.com/why-we-built-mini-elixir/