The short version:

* 'event driven' things like Node.js got popular because they use fewer resources to serve the same amount of data (generally). But for a lot of people, JS is not really their idea of a good time. Erlang (and Go) fill this niche pretty well.

* Elixir finally made Erlang more palatable to more people. Jose has done a superb job with it, because it's not just a nicer syntax, there is a bunch of nice stuff he's built into it. It doesn't hurt that as a former(?) member of the Rails core team, he has a deep understanding of web programming and its requirements.

* It is solid, solid tech. There's lots of new development happening on top of, say, Elixir, like Phoenix, but the underlying system is pretty hardened.

There are lots of demands for making things concurrent, fault-tolerant and distributed nowadays, not only in big companies but also among startups. Erlang already has those features as well as standard tools (OTP) which lets everyone to create something simply.

However among all the languages which run on top of Erlang virtual machine (BEAM) like LFE, Elixir, Efene, Luerl, Erlog, and such the Elixir has an active and bigger community, more interesting frameworks and wider adoption. So it could be claimed that it helped to introduce Erlang values to a wider range of developers.

I think there's a growing appreciation for functional style more generally (e.g. pattern matching) and Erlang's just caught up in it. Compare OCaml.

A lot of the rails community is migrating to Phoenix so I would imagine it has something to do with it.