Katie's talk explained it very well. The system they built in Haskell (which is compiled to a native binary) replaced
an older system that was in an interpreted language and was too slow. Another point Katie made was that the new system took full advantage of of Haskell's ability to hide the messy details needed to deal with concurrency.
Finally as you a probably aware, communities like Python and Ruby are increasingly talking about adopting techniques from functional programming. Thats all fine, but not all of the techiqnues of functional programming transfer easily to those languages. In particular Haskell's type system provides benefits that are not transferable without huge changes to the destination languages. Then the question becomes, "why not just use Haskell"?
Unfortunately, it seems Katie's talk was not recorded.