Well, its hard for the big two to create brand new main characters, that would have a nice backing enough to stand on their own. Take Gravity for example. It bombed, even though it was a brand new character without any ties to any pre-existing Marvel character. However, a lot of Marvel and DC characters that were created back in the 60's, 70's and 80's, those character names are established and known. Even though they aren't/are popular, their history, or the history behind the character's name can be known from either DC or Marvel's own website or one of their biga$$ info books. It's just easier, and possibly to market a established superhero name, even if the one wearing the costume is a new character. Take Firestorm for example; one of the better transitions I think.