The answer to that is really simple: JSON is a widely supported standard in many platforms and languages. Whatever the latter is, is not.
What's better than writing a parser for the latter format, is not having to write anything and just using a library, which exists in practically every language for JSON.
I'd argue, choose whatever format is best for your case. If its easy, chose a binary format for small footprint and a human readable format. But chose formats that are semantic and are able to drive the client. That's what the "R"epresentation is all about in "REST".
Of course, I even write some hobby projects in Common Lisp.
There are data interchange formats that use S-Expressions, namely EDN[1]. But JSON remains the most popular format for its widespread support, and its few data types map to most languages.
What's better than writing a parser for the latter format, is not having to write anything and just using a library, which exists in practically every language for JSON.