Interesting, my question is more though why now?
Since LISP has been around for a very long time and FP has had very little mainstream interest until the last year or 2.
1. Java is not really evolving, people are frustrated
2. Multicore is difficulte and FP claim they are better at it.
3. Complexety of ever bigger codebases
4. Java Script not longer the worst language in the world, people start to understand whats powerful about it and want to know what more is possible.
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.
I probably wouldn't use it for anything today, but in another couple years maybe. The ecosystem seems like it's reaching critical mass.