Although there's a good chance there are folks who only just encountered comics when they were adults, for most fans, it's likelier they first embarked into the medium when they were young. There are comics geared towards a wide variety of age ranges, but most tended to make the big leap when they were teenagers - and that can mean there's a great deal of nostalgia attached to the comics that got them into the medium in the first place.
Unfortunately, some of those comics just don't hold up all that well when you revisit them through a more experienced lens. In a lot of cases, what could've been a nice trip down memory lane instead turns into something pretty awkward fairly quickly, as the flaws of those old, beloved storylines of yore suddenly become just that little bit more noticeable.
Of course, there's nothing to stop readers from loving those books for being their gateway into comics, and it's likelier still that plenty have their admirers today. It's just that, the further you get away from certain comics, it's easier to notice what they got wrong - in some instances because the creator of said book has gone on the record to state just how much they don't like it.
Factor in dated industry practices, a classic ol' dollop of superhero sexism and threadbare storylines being rescued by legendary art, and it's plain to see that some of those old, beloved comics just don't cut the mustard anymore - no matter how much it pains us to admit it.