More correlation than causality. You know what also happened in the early 90s? Magic the Gathering. That certainly didn't help the RPG business.
So, this is getting very far afield of the actual topic.
Just as a reminder, the actual topic is that D&D is a flagship brand, and as a dominant flagship brand, the Powers that Be have concerns when designing it other than just "good design." To quote myself, it "has to be broadly and widely popular."
That said, the whole thing about correlation does not imply causation is always trivially true (assuming you're not doing some really fancy regression analysis). I'm sure we all know the old point about "sexual assault goes up during the warmer months, ice cream sales go up during the warmer months, therefore ice cream causes sexual assault." (Confounding Variable)
That said, and remembering that overall market data is still sketchy, I think the following can be noted-
1. The first big explosion of TTRPGs occurred in the early and mid-80s. This corresponded with the cresting wave of D&D (1e).
2. The second big explosion of TTRPGs occurred in the early 2000s. This corresponded with the second wave of D&D (3e, d20).
3. The third big explosion of TTRPGs has been occurring over the last few years (see also, kickstarter). This corresponds with the third wave of D&D (5e).
Now, that doesn't mean that there aren't other games that did well at other times (e.g, WoD in the 90s). It doesn't mean that other games aren't as good, or better, than D&D (for various personal values of "good" or "better"). It's not making any forward prognostication that this is the way that the market will always be*, either (even the famous sneezing statement was later changed from France to America).
I think that there are things you can observe from this-
1. D&D is close to being a generic term for TTRPGs for laypeople.
ex. If you are going to play some other TTPRG, and you're explaining to someone who is not familiar with it, you can probably just say, "It's like D&D."
2. D&D tends to draw a LOT of new players to the overall market, and many of those new players will later play other games and/or some of them will even design other games.
-This one is kind of a big thing. D&D is, by far, the largest driver of new players into the TTRPG market. No, I don't have data, but ... do I really need it?
3. When D&D is doing well, you tend to have a much larger player base, and this player base often is more willing to experiment with other games.
-This is purely a guess, but it feels right? Eh, more research needed.
Anyway, getting back to the main point- the primacy of D&D is neither good nor bad, and it's certainly not inevitable. But that primacy is also something that will continue to drive the design decisions when it comes to D&D.
*As a quick aside, I don't count the PF/4e as a deviation from this because I count PF 1e as D&D. YMMV.