Comic book fans cared about Iron Man and that matters the most................ original screenplay is way harder to get right than adapted because it's really hard to write a good story. And to write good characters. Having the comics to adapt from removes much of that difficulty because even if you make massive changes to the comic stories, you have the greatest hits of the comic industry as your backbone. If you want to pick the shitty unpopular characters of the comic books as the basis of your movies, all you are doing is basically being struck writing original screenplay. And it is possible to get that right, but also way more possible to get it wrong too.