It's probably safe to assume a lot of that was due to some/most of Siri being licensed from Nuance initially. I mean, who wants to talk about a new product, which most people think is brand new and entirely innovative, just to say "Oh yeah, we paid someone else to work with us to create it."

Not that there's anything wrong with that and it certainly seems like Apple has been investing in-house pretty heavily in recent years for Siri improvement.