The thing is that the types are so good that they usually act as ultra-descriptive irrefutable documentation. I've found tons of straight up incorrect documentation in semi-popular Python libraries, but you're never going to find an incorrect type in a Haskell library doc.
As an example, I was doing some password hashing stuff a while back. The most popular Python library had horrible docs where all the arguments were strings and it was ambiguous what exactly went where. The Haskell library had almost no prose docs, but the type signature was something like
Password -> Salt -> Difficulty -> HashedPassword
Literally impossible to mess up.
Personally, I'll take Haskell type-based docs over any other extant documentation that I've come across. Learning to read types effectively and quickly is a bit of a learned skill, but it comes naturally with a bit of use of Hackage or Stackage.
Also, Haskell has services like Hayoo or Hoogle where you can come up with a type signature and it will tell you anything that had that signature. Like let's say I want a function that goes through a list of maybes and returns them all if none of them are Nothing. I just search for
[Maybe a] -> Maybe [a]
I get "sequence", which is the correct answer. Good luck doing something like that with the Python or Java docs.
> why there hasn't been more uptake of Haskell for all of the apparently cool ideas that are there,
People already know how to get the job done one way or another and learning new stuff is hard. It's an unfortunate but pragmatic viewpoint for many people.