Interesting presentation on functional programming: Why Isn't Functional Programming the Norm? – Richard Feldman - YouTube
Richard Feldman poses that every programming language has to have some killer app to gain any real traction. I think for Haskell it could be Plutus. For functional programming more broadly I guess smart contracts will create a lot of need for functional programming.
Functional programming is actually quite popular in academia, science, finance, etc.
The majority of the world is not modeled strictly in mathematics however. Therefore other programming paradigms such as object oriented programming which allows us to create extremely complex systems in terms of smaller abstractions like a gigantic puzzle made out of simple pieces is more useful to a variety of real world use cases. Remember LEGO? That’s what OOP paradigm provides. Making anything you can imagine from reusable building blocks.
Functional languages typically have a lot more boilerplate and need to be reasoned about on a per module and per function basis. The combinations of modules and functions can drastically change in both definition and behavior. Thus the primary pain point is the need to deeply understand the entirety instead of just the parts. The fact that you can perform provably secure and computationally complex algorithms in a concise deterministic manner in a purely functional programming language is not sexy.
This is because 90% of people only understand the context of the word “sexy” and maybe “LEGO” in these previous musings. That link was a great trip down memory lane for the history of programming language evolution though 