Additionally, I think that Haskell really has to offer something over most other FP languages. Many features of dynamic impure FP languages have already been ported to languages such as Ruby or Python. What Haskell provides over, say Lisp, is exceptionally strong typing and isolation of side-effects. This is a quality that cannot be ported to mainstream languages easily.

So, why didn't Haskell become more popular earlier? I think that's simple: the GHC compilers has improved tremendously over the past years, Hackage has grown considerably, and there are now more books oriented at practical programming.