Woah .... so that's a big question. And I can start by answering that I've been playing a modified 1983-base GH since, um, 1983 or so. Off and on.
I think the major three factors are:
1. Swords and sorcery. The other major kitchen sink setting is more of a high-fantasy setting.
2. Fill in the blanks. FR has been expounded upon quite a lot; while GH has additional materials, TSR/WoTC hasn't messed with it nearly as much, which means that there is a lot of blank canvass for the DM.
3. Ur-D&D. Nostalgia is a heckuva thing.