Remember that Erlang was open-sourced as a last-ditch effort to save it, in 1998, after Ericsson internally switched to C++ and banned using Erlang on new products.

The sad fact is that the software industry as a whole relies heavily on mainstream languages, and not on educating your engineers to leverage the strengths of your specialty language. There's rationale to that, in that it costs a ton to maintain your own platform when you can just leverage a more public one. As a random example, why spend a ton to try and optimize your compiled language when you can... just use GCC (or clang nowadays). See: ongoing complaints about the performance of Go.

Erlang's silver lining is it found some measure of success outside of Ericsson, which led it to being unbanned there (and the banner to being "promoted sideways")