For Elixir, the main reason is Immutability. Functional programming is easy to code AND to test. Having some experience in ruby, I also have to say I really like the syntax. On web part, my guess is Phoenix framework will have the same impact as Rails had in its time.
For Go, it is harder to justify this choice. The language just feels right. It is excellent for network and real-time development, is used my many huge company (Docker, Google, Dropbox, ...). Last but not least, Go has outstanding performances.