Probably all sorts of reasons. Market/problem domain choice, strategy, IP, price, weight are all hard problems and then you get to extremely hard optics issues

Probably all sorts of reasons. Market/problem domain choice, strategy, IP, price, weight are all hard problems and then you get to extremely hard optics issues that require all sorts of tradeoffs. Solve all that, and then you need to invent a whole new UX paradigm and integrate it with whatever solution it is supposed to solve. Take everything that needed to happen for the iphone and multiply it a few times.