I'm not aware of all the details around the technical complications, but from a physics perspective, you can't make something darker and more opaque by adding more light. Therefore, an AR headset needs to be able to at least partially block light in order to make convincing images. It seems a lot easier just to go with Apple's approach of blocking all light rather than try to develop tech that will only selectively block the light behind the AR objects while allowing other light through.

The blocking all light approach also allows you to hide other potential weaknesses of a device. For example, a lower field of view is much more distracting in a pass-through AR device as you still have your full peripheral vision. VR devices will generally black out the light outside of the FOV making it easier to ignore.