I worked on OCI for quite a while, the very short and cynical answer is that Docker thought the registry was their moat for a long time and fought tooth and nail to keep it at the detriment of almost everything else. (The longer version is a bit more diplomatic.)

Even better, the OCI distribution protocol (which was Docker distribution until a few years ago) is not a static-blob-over-HTTP protocol! The blob bits are and you can route them to blob storage but you need a smart server for a few key bits of the protocol.

Back in the day I made proposals for distribution formats that didn't have these flaws and were more distributed (and previous proposals like AppC's discovery had similar ideas) but they were roundly rejected by the Docker people.