I'm only half joking. The different notations all have pros and cons, but the benefit of s-expressions are their simplicity. A list can be either an expression

I'm only half joking. The different notations all have pros and cons, but the benefit of s-expressions are their simplicity. A list can be either an expression (+ x y) but can also be a list of things like (red blue green) and an empty list can be written as (). It is basically the simplest possible structured notation. +(x y) would be fine for expressions but weird for lists of things.