Imo, one "fundamental" reason is that Haskell has a much smaller, nicer core than most languages, so it builds everything out of that core. For example, most languages have variable update statements in their core. Haskell defines at least two data types of "programs that have stateful variables". The result is that people want to see how everything is broken down into pure functions under the hood, and that means it takes more time to learn the language.