But that perhaps hints at some reasons that formats like JSON tend to win popularity contests over formats like Rivest's. JSON is a single format for authoring and reading, which doesn't address transport at all. The name is short, pronounceable (vs. "spikky" perhaps?), and clearly refers to one thing - there's no ambiguity about whether you might be talking about a transport encoding instead,
I'm not saying these are good reasons to adopt JSON over SPKI, just that there's a level of ambition in Rivest's proposal which is a poor match for how adoption tends to work in the real world.
There are several mechanism for JSON transport encoding - including plain old gzip, but also more specific formats like MessagePack. There isn't one single standard for it, but as it turns out that really isn't that important.
Arguably there's a kind of violation of separation of concerns happening in a proposal that tries to define all these things at once: "a canonical form ... two transport representations, and ... an advanced format".