Why not CSV? S-Expressions are tree-structured, and that makes it easier to handle things like whether or not someone has a spouse (or multiple spouses) or a partner, or whether they have a title.
To parse S-Expressions, you just use (read) in Racket (or any Scheme for that matter). CSV isn't hard to parse, but it's not a 6-character expression.
Why not JSON? JSON is tree-structured, so it accounts for the issues above. But, most languages that slurp JSON don't have pattern-matching facilities as powerful as Racket.
If you look at the patterns I wrote to chew up entries and spit out labels, you'll see what I'm talking about.
Finally, the outputted S-Expressions are very human-readable.
racket (and scheme) can read sexprs from a port with the 'read' procedure, which then uses the interpreter's parsing engine. loading csv or json require modules.
and it's no more difficult for C to spit out strings with some parens in them than it is strings with commas.
