What 4e had going against it was legacy of perception of what D&D is 'supposed' to be with over 30 years of previous history of how the game was played and then 4e pretty much said that 'this is a different kind of D&D, made for the modern audience, and we are looking at different sources of inspiration than what the previous 30 years had been based on' and its entire design paradigm was fundamentally very different than all previous editions... and it all came down to the public's perception of what they thought D&D was meant to be vs. how the designers thought about D&D at the time and what they perceived as overall changing perceptions of modern game design and Nostalgia by the previous generations won out.
To me D&D 4e was the pinnacle of it's game design, and while 5e is pretty dang amazing in its own right... it's truly a step back towards Nostalgia and the rest of the OSR type of gaming. You can thank 4e for the OSR, as the OSR was probably an indirect result at least of how 4e was being handled. People were simply so used to a D&D with Vancian magic and alignments and specific classes being a certain way and that old schooled feel that anything labeled D&D simply, for the most part with the majority of fans, must have those sacred cows.
I never had that nostalgia factor with D&D. I am a very much a minority when I can say that D&D was not my first rpg. My first rpg was Marvel Superheroes and then Battletech, Star Wars d6, Earthdawn and Shadowrun. Those are my background, so when D&D came out with 4e and did away with all those sacred cows that IMPO were horrible game design, I was overjoyed.
and I am still overjoyed about that with 4e. They took a chance, did something different that required people to really think outside the box, and many couldn't do it. They broke with something new, got rid of a lot of what was bad design from the previous generations of D&D, and in the process made something quite revolutionary that unfortunately also had its own inner faults that showed off its warts and possible bad game design. Maybe calling it bad design is wrong, I don't know, but D&D has always seemed like a horribly designed game that for some reason people just loved.
It's nostalgia for most gamers. For me it's not. I didn't grow up playing the game. I didn't even hear about the game till after I had played a bunch of other games and when I finally got around to playing it, D&D was a vastly inferior product in every way possible. Now for the majority who grew up on D&D, because its their first love, their first influences within the gaming hobby, and it was their guiding light into roleplaying, that nostalgia factor is so strong in a lot of gamers and they had so much fun and it influenced so many people at such an early age that no matter where gaming goes in the modern day, many of those players will still play D&D as their first priority game, their chosen game, and they want D&D to be the game they remember playing and the game they have played for twenty/thirty years and there is nothing wrong with that. And for these people, 4e was so jarring a difference, it was changed in so many ways that a lot of the people who grew up playing D&D just wouldn't/couldn't look past those differences.
4e did bring in a lot of people who grew up with the internet and the video game/MMO generation because in many ways D&D 4e was made for them. It was made for a new generation of gamer, and I don't think Wizards even thought of how the many would react. With 5e it's quite obvious that they decided to go back to the many... and those who started playing D&D with 4e see it as a step backward, and rightly so.
In the end it's all perception, its subjective, it's a factor of how important that nostalgia feeling is in some people, and with many they just couldn't get past it. Their own personal paradigms of what D&D is was so strong that anything different isn't D&D... even if it was D&D.
