This is yet another of the documented appearances or usage of the Mar-Vell character post death. 
This one in particular plays on the plot hole left open in "Death of Captain Marvel" where his body and etheral form are separated before the point of death. 
 
Why don't they bring back this character rather than use it as a legacy character?

Captain Marvel does not really appear in this comic book, much like the recent Guardians of the Galaxy #12, he is used as a tool to sell comic books rather than actually bring back the character and satisfy fans, and quite possibly sell even more comic books in the long run by giving fans what they want.

Wow. Kind of says it all, doesn't it?

Why doesn't Marvel see the poignance of the story in bringing back this character?

Alex Ross is a known fan of the Fawcett Captain Marvel, Flas Gordon as well as the unique and original Mar-Vell, Captain Marvel of Marvel Comics.

It's also interesting to note that at the end of Earth X, that Captain Marvel stayed alive in physical form.