I think one of the main reasons he should stay dead is that he died much differently than most comic characters. He died of cancer, something very real. Comics are a form of escapism, but at the same time they mirror the world. Tony Stark, Iron Man has battled alcoholism for a long time now, we've seen him fight to stay sober, crawling out of the bottle and staying on the straight and narrow was at one point a defining part of his character, he did something difficult but very human and identifiable at the same time.
Could Tony, or Reed Richards or Pym or Beast or someone create some invention to cure Tony of his alcoholism? Most definately, it's comics after all. But that would cheapen Tony's battle, it would cheapen the story and would make the real world connection he once had completely void and useless.
Same thing with the new Speedy, Mia, who has AIDS. Any writer could get rid of that, some super machine or alien blood transfusion gets rid of her AIDS. Great for Mia, shitty for any fan of the character who identifies with her and has to struggle with it in real life. It cheapens the character, the stories and in a sense the real life affliction, for the writers to care so little about something so big that they'd write it away in 22 pages.
Making light of something serious and real world like cancer, saying Mar-Vell just got better because he's special would really undermine that storyline and the credibility of comics to portray intense subjects.