I think Haskell actually has a few qualities that can potentially turn people off at different points, all of which have been mentioned by others:
* Lazy Evaluation -- usually becomes an issue at some point if the project isn't trivial.
* Lack of dependent types -- while Haskell's types system is great, this is a missing piece that proves a sore absence for some. It's not very surprising that several functional languages developed after haskell, inspired by haskell, and implemented in haskell (idris, agda, coq)
* category theory -- While you can get by plenty well just knowing a few basic abstractions from category theory, it's true that many libraries rely on highly-theorectical concepts, and that a solid portion of haskell development efforts are still tinged with an academic flavor. Just take a look at some of Edward Kmett's coding videos--he typically has one (or several) mathematics/highly theoretical comp sci papers open in one window, while in the other he's implementing a hugely popular library like lens. This sort atmosphere of academism turns a lot of people off. It pegs the language as a theoretical exercise from the get go, and plenty of people are either don't believe in the value of theory or are far too busy to take the time to dive into how it applies to their situation.
I think the academic veneer around haskell is its biggest weakness when it comes to adoption, but that's said, it's still had more success than a lot of other academic, purely functional languages thanks to the efforts of core researchers/users/contributors to break it out of the ivory tower. The ideas are good. Almost every other modern programming language has stolen concepts from haskell at this point--so in some sense even though it's still somewhat niche its influence is practically ubiquitous (just take a look at the list of languages its noted as having influenced on wikipedia! https://en.wikipedia.org/wiki/Haskell_(programming_language)).
In a broader sense, the functional programming paradigm has proven not only that it's a viable one for industry applications, but often superior to object oriented or imperative techniques when it comes to fidelity of expression and preventing bugs. If nothing else, haskell is great because it provides a fairly rigorous environment in which you can explore functional programming concepts, which you can bring with you to almost any problem and any language and manage to derive some benefit.
* Lazy Evaluation -- usually becomes an issue at some point if the project isn't trivial. * Lack of dependent types -- while Haskell's types system is great, this is a missing piece that proves a sore absence for some. It's not very surprising that several functional languages developed after haskell, inspired by haskell, and implemented in haskell (idris, agda, coq) * category theory -- While you can get by plenty well just knowing a few basic abstractions from category theory, it's true that many libraries rely on highly-theorectical concepts, and that a solid portion of haskell development efforts are still tinged with an academic flavor. Just take a look at some of Edward Kmett's coding videos--he typically has one (or several) mathematics/highly theoretical comp sci papers open in one window, while in the other he's implementing a hugely popular library like lens. This sort atmosphere of academism turns a lot of people off. It pegs the language as a theoretical exercise from the get go, and plenty of people are either don't believe in the value of theory or are far too busy to take the time to dive into how it applies to their situation.
I think the academic veneer around haskell is its biggest weakness when it comes to adoption, but that's said, it's still had more success than a lot of other academic, purely functional languages thanks to the efforts of core researchers/users/contributors to break it out of the ivory tower. The ideas are good. Almost every other modern programming language has stolen concepts from haskell at this point--so in some sense even though it's still somewhat niche its influence is practically ubiquitous (just take a look at the list of languages its noted as having influenced on wikipedia! https://en.wikipedia.org/wiki/Haskell_(programming_language)).
In a broader sense, the functional programming paradigm has proven not only that it's a viable one for industry applications, but often superior to object oriented or imperative techniques when it comes to fidelity of expression and preventing bugs. If nothing else, haskell is great because it provides a fairly rigorous environment in which you can explore functional programming concepts, which you can bring with you to almost any problem and any language and manage to derive some benefit.