There are a few problems with functional programming languages that have prevented their true adoption anywhere.
1. Limited paradigms - I always prefer languages that let me write my code the way I want, a la C++, than a language that requires a strict paradigm from academia like Lisp. If I want to use the inherent hardware property based side effects of certain code structures, let me. Programming languages =/= mothers.
2. Difficulty. 90% of programmers (not on the internet, in general) write code like Fortran when its 2010. The most popular languages now, C# and Java, are popular because they are extremely easy to understand, if not easy to get things done in. You dont need to know lambda calculus or templates or prototyping to understand 99% of C# / java code (yes, I know C# has all of those and java has 2/3 of those). The problem with functional languages is that they always use these paradigms.
3. Most functional languages except Ocaml are like Ruby and Python in that they have tremendous performance overhead. For a consumer application, that overhead usually doesnt impede adoption (its more like the software is poorly written than the applications environment is too inefficient). But when talking about server programming the costs of running something under Ruby vs C are astronomical, and the same problem arises with functional programming. It might not hurt the consumer that the Python implementation of their music player consuming 30% more clock cycles than the exact same program written in C, but it does cause huge scaling issues with popular resources like Twitter.
4. In extension of 3, functional programming is getting away from how the hardware actually works. It is good for a novice that doesnt want to get into the details of pointers and caching and disk IO, but professionals should enter the game knowing how the underlying system runs and that making tradeoffs for readability by someone who doesnt know the language anyway vs performance benefits falls to the wayside. Developer time is important, but when you factor in the massive overhead trying to get 20+ year professional developers in C to try to think functionally you are never justifying the upfront cost of using the languages.
I mean, I dont use them. Thats personal preference. I like the way C and OO work more than I like dynamic typing and having no data and all the other out of this world paradigms. I really hope that D can achieve what I hope it will evolve into, a language that is hopefully as easy to understand as Python without the boilerplate of Java but with the performance of C. Thats kind of where the end goal of programming languages needs to be.