It's not new. BOMStore is a format inherited from NeXTStep. JSON didn't exist back then.

Also, it's a format designed to hold binary data. JSON can't do that without hacks like base64 encoding.

Binary file stores like this are very common in highly optimized software, which operating systems tend to be, especially if you go looking at the older parts. Windows has a similar format embedded in EXE/DLL files. Same concept: a kind of pseudo-filesystem used to hold app icons and other resources.