That's a good observation. I don't know much about Erlang's history beyond what is on Wikipedia, but I suspect that the number of people that used Erlang before it was open source was relatively small and limited to the technical community within Ericsson. That small, focused user group explains the language's weird and weirdly antiquated style and feature set, but it doesn't quite explain why the language has not been evolved to become more modern. (Of course, the fact that something as ingenious could come out of such a small community is also very impressive and wonderful.)
