> In actual Common Lisp development, code is stored in text files and edited and diffed as text in source controlled repositories.

That's true, but there is a very big difference between S-expressions stored as text and other programming languages stored as text because there is a standard representation of S-expressions as text, and Common Lisp provides functions that implement that standard in both directions (READ and PRINT) as part of its standard library. Furthermore, the standard ensures READ-PRINT equivalency, i.e. if you READ the result of PRINTing an object the result is an equivalent object. So there is a one-to-one mapping (modulo copying) between the text form and the internal representation. And, most importantly, the semantics of the language are defined on the internal representation and not the textual form. So if you wanted to store S-expressions in, say, a relational database rather than a text file, that would be an elementary exercise. This is why many CL implementations provide alternative serializations that can be rendered and parsed more efficiently than the standard one, which is designed to be human-readable.

This is in very stark contrast to nearly every other programming language, where the semantics are defined directly on the textual form. The language standard typically doesn't even require that an AST exist, let alone define a canonical form for it. Parsers for other languages are typically embedded deep inside compilers, and not provided as part of the standard library. Every one is bespoke, and they are often byzantine. There are no standard operations for manipulating an AST. If you want to write code that generates code, the output must be text, and the only way to run that code is to parse and compile it using the bespoke parser that is an opaque part of the language compiler. (Note that Python is a notable exception.)

Its interesting that despite the utility of S-Expressions, as mentioned, semantic diff, for example, of CL code is uncommon.

By that I mean highlighting the diff between these:

  (dolist (i l)
    (print (car i)))

  (dolist (i l) (print (cdr i)))

I'm pretty sure this exists, but it's uncommon (at least to me its uncommon).