The decision is centered more around the BEAM [1], the virtual machine which powers Elixir/Erlang. It was designed for high-availability systems and it's very efficient at managing large numbers of processes and threads.
In the context of these tools, it's useful for handling open connections and spiky workloads.
edit: Also worth mentioning - we don't care too much about the language. We aim to use the right tool for the job, or preferably an existing open source tool. A good example of this is PostgREST which is written in Haskell. The only person on our team who knows Haskell is Steve (the maintainer). We don't mind because it's the right tool for the job, it's open source, and it has an amazing community.
[0] Pooler: https://news.ycombinator.com/item?id=35501718
[1] BEAM: https://blog.lelonek.me/elixir-on-erlang-vm-demystified-3205...