I think it's a combination of many, many things. You can't break it down to just one major contributing factor.
They sacrificed a lot of sacred cows. And really, I think the tree of D&D gaming needed to be watered in the blood of a few of those cows. I like 4E, and don't know if I would have if they had changed it less. But there were a lot of abrupt changes that were probably jarring for some other longtime fans or players.
Yes, the default setting is less of a setting and more a default collection of themed ideas. Which is good for the basic game, but then they carried that default collection of ideas over to all of the versions of other settings for this edition of the game. I understand not wanting to do the same thing they did last time for, say, Forgotten Realms but they basically gave it the points of light treatment of the default game. So all of the settings have a certain sameness to them. Dark Sun is different in that it takes "points of light" to the extreme, which is probably why it's so well regarded for this edition. But even there they just took the default collection of themed ideas and carried them over to a certain degree.
Those two things together would be bad enough for an established brand.
But really, I don't know about the rest of you, but the way that they present and "sell" the game grates on me sometimes. WotC relies a lot on their game developers to market the game, new products, and changes to the line.
Which is fine to a point, because gamers like to talk about games with gamers. They trust the opinion of someone who understands, plays, and genuinely likes the game rather than a stuffed suit. But there's a reason why these people didn't just become salespeople, marketing gurus in some other field, or even customer service people. They just don't seem to think about how things sound sometimes, or about how what they say is going to be parsed bit by bit and come back to haunt them. Or perhaps they're not as adept about thinking in those terms. Or perhaps it's that they are proud of their work, or utterly convinced that their direction is the best one, and so there's a degree of arrogance in what they say that turns some people off.
For some reason WotC line developers for D&D seem to me like they get stuck in this weird place between "director of marketing" and "game-developing gamer" that looks a lot like someone stuffing both feet in their mouth at the same time while inviting random people to walk up and kick them in the kidneys. I think the responsibility for promoting the game needs to be in better hands, and in other hands, rather than in the hands of the same person who is responsible for the changes the company is trying to sell. Really, for D&D, WotC handles marketing and communication like a mom-and-pop operation. The "owner/operator" of the game line is handling all of the hype and explanation of the product.
Now, combine this with my first two points, about sacred cows and not having a default setting but a default set of assumptions exported to all settings, and it's an uphill battle they're fighting. Things can very easily take a turn for the worse with these problems.