Because as soon as you adopt the s-expressions, what you got is no longer <language>, but lisp itself. Something like this:

  static char _getch() {
    char buf;

    if (read(0, &buf, 1)) return buf;

    return '\0';
  }

would become:

  (define _getchar ()
    (declare static)
    (return-type 'char)
    (let ((buf (char)))
      (if (read 0 (& buf) 1)
        buf
        "\0")))