Like any other design choice, fads and trends abound. Dark themes are on the rise, so that's probably part of it. Another part might be consistency between devices. Dark themes are preferable on the AppleWatch because OLED screens use less power the darker the screen. I've notice that the world clock on the AppleWatch has changed to the same color scheme graphics.
That's not true of iPad's and iPhones, but there are rumors (and have been for a while) that our bigger devices will also use OLED screens in the near future. So maybe its leading for that. Of course those rumors have been around for a while, so probably not. I don't believe OLED screens can provide the same pixle density and brightness as the curren screens, and Apple is unlikely to go backwards on screens quality 'just' to save some power.