But how does this fit into the narrative about how 5E is the worst, the books are mediocre, and nobody likes the game...?
Inquiring minds want to know!
I realize this was probably a bit facetious, and I don't think 5e is the worst, but....
One explanation is the power of network externalities. That is, D&D has always been mediocre, but it is the most widespread (being the first in the market) and so is much easier to find a group to play in (which...y'know...is kinda important). Once a standard like that gets put in place, its can be very hard to displace, even if its demonstrably inferior to newer improved standards. This is especially true if the imperfect standard "works pretty okay" and isn't totally trash so that a lot of users already have favorite workarounds (i.e. houserules) for its quirks. I would say that most of D&D (across editions) falls into that category.* 5e has made adoption easy, and they've gotten back to its "works for the most part" roots, that amplifies the strength of its market presence.
I'm not saying I know or believe that that is 100% the case, but I don't think its an insignificant factor. Especially since I just read an article about "10 worlds I'd love to play in a D&D campaign" that cited non-D&D games by name .... as D&D?!? That was a head scratcher for me. I mean, here's an author who was saavy enough to know that
MASHED was Powered by the Apocalypse, but still called it D&D. If "D&D" has become the word for "Role Playing Game" in the way the "Band-Aid" has become the word for "Adhesive Bandage".....
*You see that kind of thing in other products that exhibit network externalities, too:
"God I hate working with product X and all its little quirks and annoyances."
"Why does everybody keep using product X instead of switch to product Y, which avoids all those problems?"
"Because we all have product X and Y isn't 100% compatible with the other stuff in the network, plus I've written all this custom software to deal with X's problems...and it won't run on Y."
"Why do we all have product X?"
"It was the first one out there by 5 years."
Plus, there are other "symptoms" like multiple active forums devoted to dealing with the quirks and problems of product X....::looks around, clears throat::.... also grognardic cultural guardians.....::looks around, clears throat::...and I'm sure there's others, but this post is already pretty long. Thanks for sticking with it to the end.