4e tried the generic none setting and it ended up turning into the Nentir Vale setting, but lore just kept being added bit by bit.
I think this is the crux. Adventures need a setting, and a cohesive setting allows for cohesive adventures. With Tyranny of Dragons, they want the PCs to see how their actions impact the greater world, and so they need the greater world to actually be fleshed out and a part of the adventure.
The setting needs to be the generic medieval fantasy that new players will expect, and most people who don't homebrew play with. FR is about as generic as you can get (if you ignore Time of Troubles, Spellplague, and the Sundering, which to a large extent you can), it's the most popular, and it's there. They own it. I suppose they might have used Greyhawk, but there are likely many more complications with that.
People who aren't playing the adventure lines will certainly be able to ignore any FR content in the basic books. And I think people who want to play the adventures in their own worlds will be able to drop them in, with some tweaks as per usual, just as well as those adventures in "non-settings".
Sure some people hate FR. Everything is hated by some people, and some people hate everything.