There are a few problems with functional programming languages that have prevented their true adoption anywhere.
That's true (mostly; if you've bought a plane ticket in the last 5 years, there's a good chance that functional code priced your fare.)
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.
Are you from bizarro world? You can say a lot of bad things about Lisp, but "limited paradigms" is not one of them (unless you've got extremely specific criteria or don't like parenthesis.)
2. Difficulty. 90% of programmers ...
I'll give you that one, functional programming is hard to grok.
3. Most functional languages except Ocaml are like Ruby and Python in that they have tremendous performance overhead...
C's going to win on I/O and other low level stuff, but modern Lisp compilers can produce some pretty efficient code.
With all the disadvantages accounted for, there's still a reason some people use functional languages. There are a bunch of things you can do that are near impossible otherwise.