I admit to fawning for functional languages like Haskell, Clojure and OCaml for fun, but it seems like they are not more popular because having many additional, higher-order constructs makes code “obsfucated” behind clever features rather than written as simply as possible (but no simpler). Maybe for a coding shop full of insane geniuses (i.e., quants, CalTech aerospace, physicists or Stanford research department) it’s all good, but the learning curve of tight functional code is too steep in the vast majority of situations. Even if it’s good, clean and small code, functional is too often a “white elephant” because of the dearth of its practitioners. Hence the pervasiveness of C++, Java, .NET, Python and especially JS.

One the fundamental UX, copy-writing, devops/sysadmin and programming axioms is:

Don’t make me think

e.g., reduce cognitive load by making knowledge works as easy to digest as possible using simple, familiar/conventional, intentional and clear patterns.