I think it's actually a third-order effect resulting from this: being harder to learn means that fewer people learn it, which means that there are fewer libraries and less community support available for the language. That means that even experienced, highly-skilled practitioners who could easily pick up the new language in a weekend are less productive in it.
I've written Haskell, I've written Scheme, I picked up Go in a couple weeks, and learning a new functional language shouldn't take me more a few weeks. Nevertheless, if I were starting a project now, I would probably pick some combination of Python/Java/C++. Why? Because their library support & ecosystem far outweighs any productivity boosts I could get with Go, Node.js, Clojure, Erlang, Haskell, etc. And yeah, I know you can use Java classes with Scala/Clojure, but there's an impedance mismatch mapping the concepts and existing library structure onto a functional language that doesn't exist with Jython.
I've written Haskell, I've written Scheme, I picked up Go in a couple weeks, and learning a new functional language shouldn't take me more a few weeks. Nevertheless, if I were starting a project now, I would probably pick some combination of Python/Java/C++. Why? Because their library support & ecosystem far outweighs any productivity boosts I could get with Go, Node.js, Clojure, Erlang, Haskell, etc. And yeah, I know you can use Java classes with Scala/Clojure, but there's an impedance mismatch mapping the concepts and existing library structure onto a functional language that doesn't exist with Jython.