In this comment I just discussed Go, since that's what the parent comment mentioned.
Haskell and OCaml are both great languages, but to a lot of people, they come off as much more "academic" or "ivory tower," and require a lot more effort to learn to use effectively for people who are not used to functional programming.
Couple that with the fact that they still use GC, so are a lot harder to get consistent low-latency performance as well as being a lot more difficult to use for any applications where you need to embed them in other applications, and they just don't offer enough of a reason to move to them over the current combination of "Python for high-level stuff, C or C++ for low-level, performance sensitive stuff".
Rust, on the other hand, promises to be effective for the low-level, performance sensitive stuff, is good for embedding in other languages, offers the additional benefit of giving statically checked memory safe multithreading with shared mutable data structures, and provides a powerful static type system for effective programming in the large.