Step-by-step explanation:
Christopher Reeve said this because of the extraordinarily innate abilities found in superman. Superman's extraordinary abilities have been displayed from birth as against other superheroes who become extraordinary in their aging process. "American heroes are usually rugged individuals linked to the Horatio Alger archetype - plain folks from humble but often rural beginnings who react gracefully to the hands they are dealt or achieve the impossible against the odds"
Other heroes have several vulnerabilities including self-doubt, but superman has just one vulnerability- a green rock.