From what i have read, the main reason why Nvidia cards cannot use Adaptive sync is because the Display controllers don't support it. Even up to the 900 series. And the main reason for the external controller in the monitor is to provide this support as well as frame doubling for when the FPS gets too low. So although i don't like Nvidias proprietary ecosystem, using an external control system was the only way they could provide support for a large gpu lineup.
AMD cards have had an Adaptive framerate ability since the 5k series, as they have supported internal Display Port features since then. just the Display controllers did not have the full capability on DGPU's. This is the reason why GCN1.0 parts only support Adaptive sync for video content etc and not full framerate control.
But it is good that intel are supporting the open standard for the future, even if their current hardware cannot run it.
Spot on. Nvidia catered for their own and they support far more GPUs because of the way they did it. G-Sync goes as far back as the 6 series GPUs off the top of my head, which is a very good and diverse thing.