Experience. I still don't quite understand it fully, but I'm much more productive in Go than any other language I've used, and I have deep experience in about 8 popular languages and passing experience with another 12. I can list things that contribute to that, but a list of ingredients does not equate to a culinary masterpiece. Other people's experience may vary.

Being a "boring" simple language with limited options for implementation means more focus on the problem and less on the infinite number of ways to solve the problem "elegantly". But there are a lot of other factors too. Static typing helps once the code base gets above 5000 lines, for example. Fast iteration from making a change to running the tests again also helps.