Plus they wanted to take D&D in a different direction: no vancian magic, elves have shorter life spans, dwarves no longer live inside of mountains/have darkvision, dragonborn, etc. Not having the game's signature setting follow in those changes would be a rather difficult sell.
I'll add two thoughts:
While Realms devotees love the intricate history of the Realms, most gamers can't even tell the difference between Greyhawk and the Forgotten Realms (quote from Mike Mearls where he says as much).
Then there's the question of bringing in the next generation of gamers.
As someone that went from a casual gamer to a hardcore gamer less than 10 years ago, I am not interested in reading fiction or adventures from 20 years ago. So I am not going to be writing Realms material. I'm not alone on that.
That's not to say I'm opposed to research, but if I can choose writing an Eberron adventure or a FR adventure, I'd pick Eberron simply because there is much less material I have to know.
You're free to think what you want, but I don't think laziness had anything to do with it on anyone's side. I'm sure there were thousands of untold stories you can tell about the realms before. But sometimes it's nice to be able to tell the story you want to tell, and not the story you want to tell molded into shape by over 20 years worth of other stories. It's not laziness that drives that, just the desire to tell your story in your own voice.
All that stuff that happened in the realms in the past? Awesome, I can use that if I want to. I can look back on that and enjoy it if I want to. I'm also looking forward to the new stories though. The fresh ideas and original thoughts we'll see going forward.
Again with calling people lazy... You say laziness, I say too much work for something supposed to be an activity someone does with his free time.
Taken by itself, I think a lot of the Realms ideas would have been better received by some people had it not borne the FR name.
I'll add two thoughts:
Taken by itself, I think a lot of the Realms ideas would have been better received by some people had it not borne the FR name. By itself, there's a lot of "Scarred Lands" flavor to me from the areas outside the Western Heartlands and the Sword Coast - a feel of a place torn apart, and ripe with adventure hooks due to the ripping. To me, it feels like Toril has not been meshed with Returned Abeir, more like it's been meshed with Scarn -- I'm not saying it's good or bad, that's just the flavor I get from it. I liked the "set-piece" feel I got with the 1987-2007 Realms, but I can also see a lot of the appeal of making a lot of stuff unknown again.
OK so why water the FR down then?
The reason I cannot buy this argument is because there are plenty of campaign worlds out there. If one wanted to tell a story in the Forgotten Realms in the first place, why would the lore discourage them? A majour reason for the realms was its rich lore and history. If someone did not want that background why would they want to use the realms?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.