Traditional mythic heroes have a inescapable, irreversible destiny and character, that is, they have their strengths and weaknesses and a certain purpose or destiny that will always follow. Traditional mythic heroes and their stories are past, they have already developed their history and the public is certain of the outcome. In Superman comics, we are not aware of the history of this hero and therefore are more concerned with "what will happen" as opposed to what has already happened. Having a weekly release of new Superman comics can change the character's destiny and leave room for changes in his character.
"The mythic character embodies a law, or a universal demand, and therefore must be in part predictable and cannot hold surprises for us; the character of a novel wants, rather, to be a man like anyone else, and what could befall him is unforeseeable as what may happen to us."
In the same sense, a mythic hero must also follow the laws of time, whereas Superman is nearly omnipotent and therefore unable to develop as other mythological characters might. He is stuck in a never-changing present (the price you pay as a well-read comic series), where no one can hurt him but also he cannot marry his love interests, reproduce, or grow older since this could cause conflict in the series. Since time has no bearing on Superman, he evolves into the character we know him as today: an evolutionary superhero... he goes from jumping over skyscrapers, running faster than trains, and having impenetrable skin in his first appearance in 1938 to having x-ray and heat vision, super hearing abilities, being able to fly at the speed of light (and break the time barrier), while having impossible strength. His history is found after his appearance (and changed as well). He grows up Clark Kent in Smallville and the entering manhood, moves to Metropolis to become a reporter, but he is always Superman and he always tries to blend in with the normal humans. The good/evil concept in Superman comics is on a much smaller scale due to his longing for a normal human life, where evil is embodied by an "offence to private property," good is represented only as "charity." Superman could fight all evil in this world, he's certainly capable of it, but he holds to these simple civic responsibilities, I believe in attempt to stay grounded as Clark Kent.
Superman may not be a traditional mythic hero but he is far from ordinary to me. Having the ability to change his future, and even his past, writers have the most amazing task at hand. They can rewrite time and his development, superman can be reborn, nothing is out of the question or impossible for this superhero. As I read more about comics and especially superheroes, I am constantly amazed at how creative their lives (past and present) are. I am aware of the legacy that is Superman. The comics have become movies, tv shows, books... a topic of conversation and debate for decades. It is not surprising that Superman is a favorite and has lasted so long in comics and media!