The other reply mentioned tools and documentation, which are the big differentiators in my opinion, but there are also some language level tools in Elixir which feel more "modern" I guess, the macro system which is on part with Lisp, protocols, a consistent standard library which extends Erlang rather than replacing it (although it does duplicate some things, both because binaries are the default string type in Elixir, and because OTP has a bit of an annoying problem with switching the position of the subject), and some conveniences like pipelines, the `with` construction - basically it produces something which semantically is like Erlang at its core, but how you go about solving problems in it offers a lot more options, which oftentimes result in a lot less code written for the same effect.
It's more than that; there's a lot of tooling and documentation around Elixir that either doesn't exist in Erlang or appears to exist only via tribal knowledge ("Well, of course everyone knows you need to {X} to {Y}, why waste time writing it down?"). My experience trying to write a gen_server that could be upgraded on-line in Elixir was vastly simpler than trying to figure it out in Erlang.
That said, I think Elixir has helped inspire the Erlang community to get better at documentation and tooling, so that's a definite win for both.