Unofficial short answer: 'cause sexps are really easy to parse and also represent.
If you drank the Lisp kool-aid, perhaps in the salad days of your youth, you remember that type t = Atom of string | List of t list is all the structure you need to usher in the whole universe. Using sexps may bring you back to fond childhood places.
More practically, sexps make a good configuration language (especially when supplemented with Blang), the two text editors already have modes for them, and if you’re using a third-party tool/system that doesn’t understand them you can crush out an ad hoc sexp parser pretty quickly.
Be sure to get familiar with the sexp swiss-army knife tool while you’re at it. At least so you can sexp pp in a shell pipeline.