I guess the main possibilities are (1) use an existing openly specified format, for which there is already at least some published working code, even if it isn’t perfect, or (2) invent a brand new format, and hope you don’t screw it up somehow by overlooking something important. I’m not entirely convinced #2 would have been any better (does Apple have a good reputation for inventing data serialization formats?), and inre #1 I’m not sure if there’s any extant choice better than this new format with support for all of the uses Apple cares about.
Considering that all the widely supported formats throughout the industry have various severe limitations so that it’s impossible to cover the typical use cases (even for just web images, say) using less than 3 or 4 completely different formats, it’s not clear that anyone else has figured out how to really solve this one either.
Too bad that the world couldn’t find someone with good taste, extensive experience, and a forward-looking vision to make a more future-compatible image format about 15–20 years ago.