I think that part of the problem is that more 'powerful' languages often more concept heavy than more 'pragmatic' languages and that makes them harder to learn for a lot of programmers.
Take Clojure for example. A lot of people including myself like this language a lot, but to be productive in it, you have to get used to the JVM, Lisp s-expressions, a heavily functional programming style with few side effects, a significant break from a normal approach to OOP, and Clojure's distinctive concurrency constructs which are more complex than Go's. It's not that any one of these things is hard, but you have to know all of them. Unless you have Lisp, or Haskell (or similar) experience, the only one you're likely to know is the JVM, and even then, maybe not.
This isn't like C++ or Java which are huge languages with a lot to wrap your head around but at core simply reuse concepts that most programmers are familiar with, (OOP, basic types) It's the very things that make the language interesting to begin with that makes it trickier to pick up.
Most of this holds for Haskell, Erlang, etc.
Then again, real code is written in these languages, but they don't tend to dominate.