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.
I think it has more to do with the fact that they are finally starting to allow their researchers to publish. They were platinum sponsors at INTERSPEECH 2017 this week, and actually published a paper there. I'm pretty sure that was the first time _ever_ despite their recruiters showing up every year.
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.