Well, you could ask “why not?”
Erlang and Elixir have so much in common, the differences are almost negligible.
One of the advantages of Erlang is that the community is full of hardcore OGs who have been woking on distributed systems for 30+ years. The level of knowledge and experience there is absolutely fascinating!
I don’t look at it as “one or another”. It will tremendously help any Elixir developer to learn Erlang and dive into Erlang resources. Will make you better Elixir practitioner too.