While I am a WW fan, most people can't relate to the characters. She's just too foreign and few creators "get" her.
Superman may be an alien, but he is a messianic figure, who embodies hope. He had an American, midwestern upbringing and even to non-christians his values are familiar. He is a man out of place, who lost a family/people that he never got to know, and who seeks to connect to his heritage. He is also alone even though he has people who care for him. These are all things that most people can relate to on some level.
Batman is the very embodiment of loss, tragedy, alienation, the battle with inner demons and the desire for vengeance.
Both characters are fantastical representations of the "masks" that most people have to wear. Everyone must be different in different situations. They are also both representations of mankind's desire to make their environment safe and to have the power to fight those that are more powerful.
Wonder Woman has very little of that.
First she is a woman and unfortunately many people, including women, are prone to see strong women as a threat, not role-models. Most societies on this planet are patriarchal and while views are still evolving and maturing, the image of a powerful woman is still not as desirable as a powerful male savior. Secondly, she has little that the audience, mostly men, can relate to on a personal level. She's clay, given life by greek gods. Even among the non-religious this concept is someone "foreign" in many cultures where the Abrahamic religions are prominent.
Lastly, with the exception of the current storyline, WW is a princess, who regularly chats it up with gods, she has a "family" that loves her and she isn't out for vengeance. She is trying to assume a messianic role as a peace ambassador, but even that is not something most males(the majority of readers) can relate to. She does have the battle between her warrior nature and her desire for peace, but that's about as close as she gets to seeming human and sometimes that seems forced. She's just a little too perfect.
Batman, Superman, Spider-man, Capt. America, Iron Man, Hulk, and countless other characters all have very human inner and outer battles. Money, love, loss, rejection, desire and isolation are all traits shared by the more popular (Iconic) characters. That's why people can relate to them.
If you ask the average non-comic book reader to tell you about Superman, Batman, Spider-man and Hulk (Thanks to the movies and cartoons you could possibly add Wolverine, Iron Man, Fantastic Four, Ghost Rider, Blade and Green Lantern) they will hit on many of the very human traits that have helped these characters survive for so long. The traits and fears they will hit upon are are almost universally shared by all people.
Now if you ask them to do the same thing with Aquaman, Wonder Woman, Thor or the Flash you will get completely different answers. Wonder Woman will likely be associated with being sexy/wearing a sexy outfit, magic lassos, and invisible planes. With Aquaman you'll get swims, talks to fish and maybe the color of his costume. With Thor you might be lucky to get someone to mention his hammer and with Flash you'll get "he runs fast" and maybe the color red.
There is a reason characters like Wonder Woman are not as successful outside of the realm of comics. Even in comics Wonder Woman is the only one of DC's trinity that doesn't have more than one title. I could be wrong, but I think that she barely sells enough to support even that one.
I'm not bashing her. Hell, she's one of my favorites, but honestly most little girls would rather grow up to be Supergirl than Wonder Woman.
If her current storyline had been her origin or were to become the definitive WW, then this might change. As much as people may dislike the costume change, the current direction of her book has made her much more human than before. I for one hope it continues. It could be exactly what she needs to truly be the icon she is supposed to be.