It's comics - the character will come back as soon as a writer pitches a good idea for him\her
They don't have to, but different kids like different things - furthermore, I wouldn't let my pre-teen nephews\nieces read the current incarnations of many of the comics you mention, some are too violent others too complicated for them to appreciate properly yet; Marvel\DC comics' market has aged and the comics have more mature themes (I don't mean just sex\violence) than they did when I was a kid.
As a former children's librarian, I understand what young kids like - marvel has virtually no presence in that area.
Here's a good list for children's comics:
http://www.bleedingcool.com/2014/02/...ght-after-all/ - Neither Marvel nor DCare anywhere tobe seen, they *do* need to up their game when catering to younger audiences and comics in general need those younger audiences if they are to survive as a medium. At the moment, Indies are doing a lot better than the big two.