Honestly, “Why?” is the first, not last, question you need to answer, not least to help clarify your own thinking and nail down your end-goals. Who’s its market, and what problems do they have that this solves? Otherwise it’s just one more solution in search of a problem, going nowhere much.
In a nutshell though: JSON (for example) doesn't mean anything. It has no semantics - it's just syntax. I wanted something that had a semantics. Plus, proper support for binary, records, and sets, and keys in dictionaries that didn't have to be strings.
There's a wee proto-essay on "why not just use JSON" in an appendix: https://gitlab.com/tonyg/preserves/blob/master/preserves.md#...
There's also a note on why not SPKI Sexps here: https://gitlab.com/tonyg/preserves/blob/master/preserves.md#..., but it doesn't render in the rendered markdown because there's a bug in Gitlab's markdown renderer: https://github.com/github/cmark-gfm/issues/121 (see also https://gitlab.com/gitlab-org/gitlab-ce/issues/52013)