The scale at which Bitcoin exchanges operate at is pretty impressive. Dismissing that due to Mt Gox's poor security is ridiculous. Plus, I don't think the exchanges you're thinking of used Erlang/Elixir.

Why not? New things tend to use new tools, so it's understandable.

We're not talking about MtGox here, which incidentally, was written in ::cue disdaining programmer voice:: PHP ::end cue::

Languages take a while to get traction. Sometimes a long while.