There certainly wasn't a dearth of RPGs at the beginning. There was a vibrant hobby community! And not just Tunnels and Trolls and Rune Quest and other D&D-type games, but also sci-fi games and superhero games and dystopian sci fi games and so on ... and this is just the 70s.
Yep, there was a flurry of 'em. And it never /really/ stopped. It was replaced with a flurry of d20 games at the turn of the millennium, but there's always some hopeful designer, publishing some brilliant innovative system, and watching it win an award or few, inspire a handful of zealous converts, and then vanish into obscurity while D&D marches on.
So I think it is profoundly unhelpful to simply say that D&D is just some complex, niche game that somehow lucked into market dominance and by the powers of pure luck and an undiscriminating and nostalgic population kept it, because that does a severe disservice to the people who play it.
OK, I'll admit, I'm cynical, I don't often think the best, nor even the second-best, nor even the mediocre of people. It's a deep character flaw and one that I work on - off-line, with little success so far.
And, yes, it's not helpful, /because of how it paints people who play the game/.
It's also not helpful, because it's absolutely true. D&D /is/ a quixotic, narrow, primitive TTRPG which, had it not been first to market, and had it not gained notoriety in the 80s, and had any number of other perfect storm factors not swirled around it, would be as niche and unknown as the Barbara Cartland Romance RPG. And understanding that really doesn't help anyone with anything - it just indulges my impulse to cynicism.
Anyway, this doesn't mean that D&D is the best because it is the biggest, but it does mean that it is more helpful to understand the things it does right in order to how it is the biggest, and why it has proven so impervious to challenges over time.
Sometimes the thing a product does right are a lot less important than things that go right. ("Better to be lucky than good," as they say.)
There will always be a other RPGs, thankfully, but no game has ever seriously come close to toppling the D&D/PF hegeomony, and I think it is helpful to understand why instead of chalking it up to nostalgia, indoctrination, and stupid consumers.
If you want to understand /why/, you really have to take into account all the explanations, including the cynical marketing & business reasons, not just the ones that might be interpreted to paint it in a positive light, or make our inner hardcore rule wonk sit up and debate.
Uninformed consumers, group-think, teen suicide, market forces, nerdrage, nostalgia, controversy - they all play a part. d20 instead of d%, irrelevant. 6 stats instead of 4, irrelevant.
And, most significantly, we should not offer up qualities as an explanation /that aren't real/. Like every time someone says D&D is "simple" I want to lock them in solitary with a 1e DMG until they can correctly recite, understand, and interpret every rule in it, resolving all apparent & actual contradictions.
..but housing lifers is /so/ expensive.