The reason, I think, is because dying was the best thing he ever did.
Though a lot of good creators tried mightily, Mar-Vell was always a dull character. He sometimes had interesting things going on around him, but he himself wasn’t all that engaging.
But the story in which he succumbed to cancer was moving, it was relevant, it was real. Suddenly, a character for whom very few felt anything (apart from admiring his cool costume design) was generating empathy from the readership.
And the longer he stayed dead, the more his legend grew, the more people became interested in him.
If you brought him back tomorrow, people would take notice—and then, very quickly, they’d likely grow bored again. Because he’s no more interesting today as a character than he was in 1981.