I had a longer comment, but I think it all boils down to this:
- Long-term fans like to be reminded of the stories that got us hooked (our own personal “golden age.”)
- A lot of the writers, artists, and editors making comics started out as fans, especially since the 1980s.
- The industry does not want to lose readers. (Maybe they don’t know how to replace them with new readers, or don’t want to chance it, or maybe they’ve just prioritized keeping the current readership over bringing in new people.)
That said, some stories are a lot more accessible than others – even with the same character and the same writer. Geoff Johns’ Flash: Rebirth miniseries was steeped in 60 years of Flash mythology. His first story arc on the new ongoing pared it down and made a point of establishing everything you needed to know for that story as if it were just being introduced for the first time.