The way I like to think of it is that a large component of PL design is making something pleasant for humans to use, which is more of an art than a science: inherently subjective (not everyone likes the same things), holistic (it's not any one feature of Python that makes it Pythonic, it's the whole philosophy), and hard to quantify. So it's no surprise that rigorous design is not the prime factor in popularity of programming languages.
I don't think, however, that really explains why research languages these days have very little in common with popular languages - why purity and strict types and all their baggage, despite their promise to make software less buggy, have not been widely adopted. There are a few reasons why this might be the case (not just "FP is hard and most programmers aren't very good at programming", although that is one factor), but their sum seems to be something fundamental about software design - perhaps changeable, but identifiable as a unit, not simply a consequence of the limitations of rigorous design.
I don't think the divide between research and industry languages is as deep as you imply. Take your examples: purity and strict typing. Purity has been adopted wholesale by Clojure and has influenced C++11 (constexpr implies purity in addition to many other things). Go, the subject of the post, is stricter about type conversions than the C family languages are. Java's type-safe implementation of generics was influenced by ML.
> why purity and strict types and all their baggage, despite their promise to make software less buggy, have not been widely adopted.
I am guessing is it mainly due to the inertia of the current popular languages (C, C++, Java, Python, etc.), and that none of them offer such guarantees. There are tons of libraries written in them, and so it is hard to move away from them. I am hoping that Scala and Rust will change things.
