These characters were deemed unnecessary because either
1) they're literally the same as the original superhero or
2) they just play "wife" to the original.
Basing a superheroine off of these two things will inevitably make for a very weak character. If these characters were more complex and unique from their male counterparts, I believe that they would be a lot more successful. However, it's become an expectation for a female version of a female to be more of a supplementary "girlfriend" character, which is why I believe, for the most part, unique superheroines without a male "origin" make for more legitimate characters in the comic book world (when compared to stereotypical female 'clone' heroines).
Side note: Why would a superhero want to marry an exact female clone of themselves? The idea is both narcissistic and really gross.