I suspect this is partly due to the quality of documentation for Elixir, Erlang, and BEAM. The OTP documentation has been around for a long time and has been excellently written. Erlang/Elixer doc gen outputs function signatures, arity, and both Elixir and Erlang handle concepts like function overloading in very explicit, well-defined ways.

Thats a large reason for sure!

I'd layer in a few more

* Largely stable and unchanged language through out its whole existance

* Authorship is largely senior engineers so the code you train on is high quality

* Relatively low number of abstractions in comparisson to other languages. Meaning there's less ways to do one thing.

* Functional Programming style pushes down hidden state, which lowers the complexity when understanding how a slice of a system works, and the likelyhood you introduce a bug