I hate to put it this way, but I think it’s a case of catching lightning in a bottle. It’s not enough to have a good costume design (in fact that might not even be necessary. Wolverine is a pretty iconic character, and his costume isn’t all that great. A lot of times he isn’t even wearing it.), or cool powers (case in point: Batman and Captain America have none). It’s more about having something about the hero that resonates with the readers, a sustained level of excellent writing to give the character longevity, and maybe a few factors that I’m not even thinking about. Making a great/iconic character is something that takes years to do, and I don’t think there can be a single attribute you can point to that any of the characters have in common.