It's more of a strategic answer than a technological one. Per usual, Apple is taking a different user experience approach to the problem. It would be difficult for Google to mimic as their main monetization driver (Adwords) depends on their current model of search.
As far as their NLP technologies, I don't know enough about what they respectively have under the hood. I will say that Siri's ability to understand a variety of human phrases and contexts was more impressive than anything I've publicly seen from Google.
