I think there's a reason why none of Ron Edward's games have sold anywhere near the number D&D has... and that's because most people have never even heard of a game by him. The fact that every game store that carries RPGs carries D&D and everyone who has played an RPG has heard of D&D is the primary reason D&D sells more.
I wouldn't say distribution is the primary reason D&D is more popular than indie games. At some point design aficionados are going to have to recognize that D&D has genuine, fundamental appeal as a game. And Ron Edward's games aren't even played by storygamers more than once or twice. That's because most indie games have a very narrow focus, and their main aim is to impress other game designers. In some senses, the indie RPG scene is like the eurogame scene - the hardcore fans like to try 30 or 50 new games a year because they love learning new systems. They'll praise a designer like Martin Wallace to the skies, even if they've never played any of his games more than three times.
D&D is flexible and broadly appealing enough to sustain its appeal over dozens and hundreds of sessions. That has tremendous value to the gamers out there who don't especially enjoy learning new systems - and they are legion.
What I found astonishing about the whole Essentials debacle was that there was no book actually called a PHB that you could point people towards. (And, yes, I realise there still was the original 4E PHB but it had so much errata that a new one really was required.) The takeover of TSR by WotC and the subsequent professional business analysis that was performed revealed very clearly that EVERYTHING had to drive sales of the PHB. The advent of DDi has not changed that.
Frankly, it was just a half-assed revised edition that, at the time, seemed to be a way to buy a few more months for the edition and which was then subsequently revealed to be exactly that: a way of stretching out the inevitable. Honestly, though, who in their right mind would publish an effectively new edition of D&D in paperback using a book size that D&D had never used before?
It made no sense at the time and makes even less sense with the benefit of hindsight.
That's funny, because I only got into 4E last year with the Essentials line, and one of the things I found appealing about it was the book format. Digest sized paperbacks are much more convenient to use at the table than big hardcovers. I've never had a group where every play had a PHB. Typically, there are only two at the table, and one of them is mine. So the split PHB books, each half the cost of a full hardcover PHB, made perfect sense to me. I was frankly disappointed that WotC went back to the old format with 5E. I still cling to a hope that something like the Essentials Rules Compendium, in the same softcover digest format, will eventually be released for 5E.
I like the Essentials format so much that I went whole-hog and bought all the Essentials books. The DM's kit is great - organization, advice, adventure, the whole shebang. The Rules Compendium is fantastic. The Monster Vault is the best of its kind. Throw in some Essentials era adventures like Gardmore Abbey and the Harkenwold adventures,, and I have a comprehensive system and excellent support that I can come back to for years. Whatever its reception by the 4E player-base at the time, as someone approaching it with fresh eyes I loved Essentials.
Cultural relevance and compatibility with current culture would be big ones. Does D&D resonate with younger people well? These days when I talk to younger PLAYERS they have basically never read, often never heard of, the literature which directly inspired D&D.
Most of the kids who started playing D&D in the 70s and early 80s weren't familiar with that literature either. And you know what? It didn't matter. D&D is its own genre. It is not a game you play to emulate books and movies - it's a game you play to play D&D. And I think kids get that, and get it more easily than older gamers who are trying to be sophisticated by playing out a schlock fantasy novel.
But I think that is consistent with what Jer is saying about 5e - it is not 4e-esque, it is Essentials-esqu with the 4e-isms that Essentials retained stripped away (or, rather, covered over in layers of obfuscation).
Essentials was an effort to redress the mistakes WotC felt they made with 4E. It was driven by the same motivation that eventually spawned 5E - a need to make the game both more familiar to older D&D players, and easier to get into for new players.