Brand brand brand brand brand.
Why was WotDQ set in FR? Why did they bring back the companions? Why DIDN'T the spellplague kill off Drizz't?
Brand.
D&D as a brand > D&D as a game (in the minds of WotC and Hasbro and their investors...and in the dollars in the marketplace). They could've retold the War of the Lance when the released 5e, but that wouldn't be a smart choice because the FR brand is stronger than the DL brand.
I wouldn't be surprised if the convo about the core elements of WotDQ went something like, "Okay. Our game is called Dungeons & Dragons, so we're going to have those as our first 5e launch. Also, our most popular world is FR, so that's where it is going to be set. So stick some dungeons and some dragons in FR and give us a plot."
The biggest, baddest dragon is Tiamat, so she gets put in FR because D&D. Temple of Elemental Evil is probably a well-branded dungeon, so that gets put in FR, too.
This isn't a bad business decision, and it isn't even an awful game decision (WotDQ is a solid adventure, and it doesn't wreck FR to have it there, it doesn't wreck the story of WotDQ to set it there). There's trade-offs, but they're kind of minor (like, they're not going to publish the War of the Lance adventure hardback next year...but they might the year after that!)
It's weird for those of us who know "A war against dragons? Dragonlance! Temple of Elemental Evil? Greyhawk!", but for most people they're just like "Cool, there's dragons and there's dungeons in the world of that book series I liked when I was 13. Neat!"