Roughly that it isn't shit.

I say that a bit tongue in cheek, but really: BSON has a monstrous amount of basic data types, some of which make very little sense and are very difficult to round-trip through other formats; and MessagePack had crippling basic sanity issues with what-are-bytes-vs-strings in its early history and that's something you simply can't recover from without declaring game over and making a new spec.

CBOR has neither of these issues. It's almost completely isomorphic to json, plus supports byte arrays. It's very nice.