* Elixir sits on top of the BEAM (the Erlang VM), which has several decades of being battle tested.
* Elixir is at its core a functional language, with immutable only (not just by default) data, and comes with built-in processes and OTP, a library that provides ready-made abstractions on top of processes. It's very, very good at concurrency.
* Elixir has Mix, Hex, and ExUnit, which provide great tooling and package management.
* Elixir's ecosystem is rather vibrant and active. Phoenix, LiveView, Livebook, Nx, Axon, and more. Elixir generally takes the approach of building things on top of Elixir from scratch, which frees it from having to deal with impedeance mismatches. See Livebook (the Elixir notebook solution) and Nx/Axon, and all the other machine learning stuff going on in Elixir right now.
I highly recommend the presentation The Soul of Erlang And Elixir by Sasa Juric.
https://www.youtube.com/watch?v=JvBT4XBdoUE
It really gets at the core of what makes Elixir and Erlang special, but I'd say Elixir has a lot more quality of life improvements over Erlang.