Paraphrasing something I read today that blew my mind:
Siri’s problem is that it’s done by Apple. And be default that means it’s going to be a walled garden with very little of the sci-fi esque, inter-software-agent communication stuff that we’d love to see.
Google on the other hand already has a great voice recognition engine and one of the best AI teams on the planet and a voice search baked into android already. On top of this, they also have a whole bunch of cloud services like Calendar, e-mail, flight booking etc.