I don't think this premise is correct. In the late 70s/early 80s, the second-biggest RPG after D&D was Traveller. Then RuneQuest.
And there's at least an argument that Glorantha is a counter-example to
@Aldarc's thesis, being a very detailed setting based on a close reading and engagement with realworld mythology and history, but one of the most enduringly popular of RPG settings.
....
I would agree with you that the premise is incorrect, but I think it has more to do with the investment of time and effort by the DM and group to make other settings successful.
D&D had a really low bar to DM, get some graph paper, draw some rooms and connecting corridors and populate with monsters, treasure and traps. They even had rules/an algorithm for it in Basic D&D and most tables would be happy.
The setting imposed no restrictions. It is Disney medieval with no effective authority. If the DM or groups wants actual Medieval social structure, go ahead, Classical world, knock yourself out.
RuneQuest is the Anti-D&D, it is for those people that took one look at D&D world building and said "that would never work" and I never played Traveller but I think that the character background stuff was a key to its success. I have made use of detail back story generators before and they can be a) great fun in their own right, and b) really add to a campaign because they tend to generate plot points, question marks in the characters background that can be used to tie the PC's in to the plot.
The other thing, the default D&D playstyle - murderhoboing, if you will, does not work with many settings. It works in D&D, the Old West - the Movie version, Certain times and locations in Traveller (If I remember the lore correctly), Post Apocalyptic. Ironically it does not work in a realistic medieval Europe - too many local authorities. Unless one of the PC is a local noble, in which case the campaign rapidly escalates to a wargame.