Greetings!
Hmmm...I think it just takes different kinds of people. What particular game they happen to like by itself isn't necessarily a reflection on their intelligence or intellectual abilities. I have some very intelligent friends, for example, that just aren't interested in playing any kind of roleplaying game. I have friends that love GURPS--but also enjoy Warhammer, and D&D. Other friends like Rolemaster, Warhammer, and D&D. Still others--perhaps most, I'd wager--strongly prefer D&D. Most of which prefer D&D 5E, though a few like AD&D. Currently, I would say 90% of people near me prefer D&D 5E. Accessibility, general popularity, positive exposure, ease of play--these are all absolutely enormous factors. Getting most people into some obscure game--or some forgotten game from decades ago that no one has heard about--just does not seem appealing to them in the slightest.
Accessibility has several factors as well. In two of my local game stores--which remain popular and huge centers for people to get together, play, and also look at games and buy stuff--D&D 5E is really the only game going. That's just the truth. Shelves and shelves of game books, mas, accessories devoted to D&D 5E, with *all* other game systems getting a half of shelf, maybe slightly more. They are entirely easily overlooked and dismissed.
People--not just the typical Grognard--but also cousins,kids, wives, girlfriends, girlfriends of your wives and girlfriends, and so on--they too, want to play what is *popular* and what everyone is talking about, and laughing, and all excited about. People are typically engaged in telling crazy fun stories about their D&D characters, playing D&D 5E--not some other indy, obscure game.
That's just the way it is.
Semper Fidelis,
SHARK