You need margin breathing room for a major component cost increase, which most hardware companies don't have. Plus you want to start small, so you can minimize

You need margin breathing room for a major component cost increase, which most hardware companies don't have. Plus you want to start small, so you can minimize panel area lost to defects. When you have big margins like Apple has on iOS devices, you can afford a small hit ramping up expensive tech high high DPI displays. Prices come down over time and yields go up, so you can make larger (iPad) and larger (retina MBP) high resolution panels taking advantage of your equipment and experience making small panels. Other companies are catching up (have caught up?), but relatively poor scaling in Windows 7 made it more difficult. Windows 8 still has the same scaling issue for desktop apps.