Joe Rossignol, writing for MacRumors:
Cue continues to oversee the iTunes Store, Apple Music, Apple Pay, Apple Maps, iCloud, and the iWork and iLife suites of apps, and handing off Siri should allow him to focus more on Apple's push into original content.
Interesting to see more and more being taken off of Cue’s plate. First the App Store in 2016, and now Siri.
Some people will likely say it’s due to Apple’s dissatisfaction with Cue’s management, but I can’t imagine that’s the case. Siri has become such a core part of all 4 of Apple’s operating systems, it makes more sense for Federighi to be have responsibility.
Also, Cue has always been known as a deal maker. Managing content deals for iTunes, Apple Music, iBooks, and now original programming has always seemed more in his wheelhouse. Though hopefully the new original content works out better than Planet of the Apps and Carpool Karaoke.
I think this is a positive thing for Siri and Cue.