In trying to figure out why Haskell hasn't been widely adopted, I think it's very hard to disentangle legitimate criticisms of the language (unpredictable performance, overcomplicated and bizarre APIs, etc.) from the simple fact that learning it with an imperative background is like starting from scratch, even for experienced programmers. This is true even if you look at the base language, without involving Template Haskell or any of the weird language extensions.
Ultimately, asking programmers to relearn how to be programmers (or, more relevantly, asking businesses to accept that overhead and/or a vastly reduced hiring pool) is just too large of an ask for whatever marginal gain you might expect from using a "superior" programming language.
To be clear, I learned Haskell for fun and got to what I'd consider an intermediate level of expertise. It was an incredibly enriching experience, and gave me a new perspective on programming in general. But it was hard, and the rabbit hole just keeps going---sure, you can juggle monads in your sleep, but are you proficient with the `lens` library? No? Well, I can recommend you a pile of textbooks that might help.
Ultimately, asking programmers to relearn how to be programmers (or, more relevantly, asking businesses to accept that overhead and/or a vastly reduced hiring pool) is just too large of an ask for whatever marginal gain you might expect from using a "superior" programming language.
To be clear, I learned Haskell for fun and got to what I'd consider an intermediate level of expertise. It was an incredibly enriching experience, and gave me a new perspective on programming in general. But it was hard, and the rabbit hole just keeps going---sure, you can juggle monads in your sleep, but are you proficient with the `lens` library? No? Well, I can recommend you a pile of textbooks that might help.