The SRD helped dramatically, and the creation of the OSR. As far as indie and homebrew development go, it seems we're in the best age now as things have dramatically matured since 2000.
For one thing, I'll admit that I'm not 100% familiar with all the places stuff gets published. It wouldn't surprise me at all to learn that there was some corner of the internet veritably blossoming with new settings in zines and documents. Also, if that is true, please point me that direction, it sounds awesome!
I also agree that these days are better for indie publishers of D&D content no matter how you slice it, even if for no other reason that now there are way, way more consumers for what they are producing than ever before. More than that, it's easier to self-publish yadda yadda, all that interwebs jazz. But it's an unbelievable boon for the hobbyist publisher to actually have a chance at finding an audience.
Part of the reason I think people look back and see the "good old days" is that, back then, the possibilities were wide open. You could publish a setting with crashed spaceships in a sword and spells fantasy dungeon, or space elves fighting brain eating squids crossing the planes on ships shaped like seashells, or a City of Doors at the center of the multiverse, or a post-apocalytpic desert roved by mutants, or whatever. The doors were wide open, and things got WEIRD.
These days the market has solidified where D&D represents a more specific and consistent brand of pulpy fantasy world, reinforced by decades of novels, video games, and pop culture. So experimentation has died down from its old, unbounded excesses, but the plus side is that the ground D&D does cover it does so better and more elegantly than in the past (IMO).
Also there is a bit of good ol' fashioned nostalgia for how things used to be too! I'm sure that's part of the equation...but when you are talking "Golden Ages", I think some amount of lionizing the past is apropos.
