(biscuit author here) I tried a lot of different formats, especially looking for something that generated small tokens, that could have a canonical form, and that was supported in a lot of languages. The canonical part was a dead end as most formats did not support that well. Protobuf ended up being the one which generated the smallest tokens, and was easily supported in most languages, even if, like you, I don't agree with everything in the way it's serialized.
Self describing formats were a dead end as well because it grows the token size
