Because Apple uses h.264 in all their hardware. They have implementations on firmware so they could accelerate video on their iPhones, iPads, iPods, even making

Because Apple uses h.264 in all their hardware. They have implementations on firmware so they could accelerate video on their iPhones, iPads, iPods, even making specific hardware for it. This makes this videos to perform great on their hardware, fast and taking less resources(more battery),it lets them make things like editing video in real time on your phone, but only works for one codec. If they use software codecs, all their competitive advantages are lost on mobile.