In my mind it's some combination of:

- New product line (this would be easier in a well established product like iPhone, Mac, etc. or from Quest 2 -> Quest 3). A lot of decisions have to be made much further in advance because you're starting from a clean slate and have to have a final product at the end of similar quality to the iPhones that have been iterated upon for 16 years.

- The Vision Pro is much more constrained in a few key areas that are hugely impacted by the addition of another display: size, energy draw, compute, weight. Much more so than something like a Mac would be. If an additional display wasn't in the budget 2 years ago then you really aren't gonna just find the space for it in all those key areas.

- Custom silicon: Any feature that requires a decent amount of compute and/or IO bandwidth will have to be accounted for when designing the chips. Meta's headsets use off the shelf Qualcomm GPUs and they could in theory bump up a level later in the design process if they need more hardware (not as easily as I described but still possible). Apple simply doesn't have that option.

- By virtue of this being Apple and not Meta/Google/Microsoft/Amazon. I'm not knocking those other companies, but they are differently positioned in the market and are ok releasing more varied and less polished products just to see what sticks. Apple enters later in the game with a product that has had more time to be refined. Google Glass, Oculus, HoloLens, etc. all paved the way for the Vision Pro and it wouldn't really work the other way around in my opinion.