Originally Posted by
Revolutionary_Jack
Wonder Woman certainly. But The Flash and Green Lantern? Well their costumes and props are iconic but the people behind those costumes aren't so. For a lot of people The Flash is Wally West, for others it's Barry Allen. For a lot of people GL is Hal Jordan, for others it's Kyle Rayner, for others it's John Stewart.
First and foremost, luck. Which to be fair, everyone needs a good deal of and without luck even Superman and Batman would have fallen.
Second, comics from the '60s to about the 90s or so, had a higher readership than any time after that. So that meant that titles could breathe longer. So even if Iron Man never broke into mainstream in the '80s with the big boys like X-Men and Spider-Man, he was still able to dodge cancellation. David Michelinie wrote Iron Man and then moved on to writing Spider-Man. Usually when writers do a run on two or more characters you tend to see crossovers (sometimes a little, sometimes a lot) between them, but in Michelinie's entire run on ASM, he never once referred to or introduced Iron Man or did anything like that. That indicates how low IM was compared to Spider-Man in that period.
Third, Iron Man is a big-league Avenger, next to Cap and Thor, so he was important in the wider continuity sense as a member of a super-team.
Fourth, editorial interest. Iron Man was given many second chances and many college tries. He was a character Marvel entirely owned and as such wasn't subject to disputes like with other Marvel creations, so that made Marvel invest a great deal in keeping him afloat, should Kirby and Ditko get the lawyers. IM was given support that for instance wasn't always extended to titles with say a female protagonist or a POC and so on.
Fifth, Iron Man's comics were never entirely great but they weren't outright bad, so they managed to provide readers a good steady consistent supply of the same old and same old, a bit like Garfield (which aesthetically is not considered a great comic strip at all). So that helps with longevity too.
Longevity like everything else isn't some objective criteria for excellence. The fact is that the history of comics is filled with great titles and characters that never got a fair chance, never got marketed, or had owners who didn't understand the situation. Take Fawcett's Captain Marvel, in the '40s he was the biggest superhero in the planet, outsold Superman, innovated in multimedia adaptations and yet a poorly arranged legal battle led them to drop out of the hero business. Had Fawcett had better lawyers, they could have defended and kept a hold on their property and who knows, maybe Superman goes under and Billy Batson becomes the cultural touchstone for a superhero that Superman only really became in the '50s.