It's an emperical observation. Compare the popularity of languages, and then compare the "difficulty" of using them.
Php, javascript, java, C# : all very popular.
Haskell, clojure, Lisp, F#, APL (and derivatives): all very niche, and requires some knowledge that one might consider "higher level" than required for the former list.
Php, javascript, java, C# : all very popular.
Haskell, clojure, Lisp, F#, APL (and derivatives): all very niche, and requires some knowledge that one might consider "higher level" than required for the former list.