Apple never uses half baked technologies, instead it implements those when everything is ready and polished…. HEVC (H.265) is not finished yet, it still doesn’t render videos with grain as good as H.264/AVC does. And videos with grain are almost 9/10 of all movies ever shot because it is the side effect of the film itself and only movies shot on digital cameras don’t have grain. So, it means that 4K videos is not yet ready for “everywhere use” at Apple. You can’t see these rendering problems on iPhone screen, but you will notice those in a second on TV.