Now, personally, I don't hate the Forgotten Realms. I first cut my D&D teeth on Baldur's Gate, Icewind Dale and Neverwinter Nights, I have a fondness for the setting. But, I can understand why people complain about its prominence and even bash it. Beyond any personal arguments that've been brought up before this point - overabundance of canonical heroes, mostly - there's one big issue.
The Forgotten Realms is tired.
Faerun has been released in every single edition of D&D. There are more Forgotten Realms splatbooks in total than there are for almost any other two settings combined, saving Dragonlance, which was the only one with equivalent popularity in its time. And almost every single one of those splatbooks can be found or is already in a player's possession. There's not information overload, but it's just overdone in terms of things to use.
In the three years 5e has been printing, we've had Hoard of the Dragon Queen (set in Faerun), the Rise of Tiamat (set in Faerun), Princes of the Apocalypse (set in Faerun), Out of the Abyss (set in Faerun), Sword Coast Adventurer's Guide (a campaign sourcebook dedicated to Faerun), Curse of Strahd (set in the Demiplane of Dread), Storm King's Thunder (set in Faerun), Volo's Guide to Monsters (heavily Faerun flavored), and Tales From the Yawning Portal (setting neutral). And ahead of us we have Tomb of Annihilation (set in Faerun) and Xanathar's Guide to Everything (heavily Faerun flavored).
For comparison, within the three years of its initial printing, 4e gave us campaign & player's guides for Forgotten Realms, Eberron and Dark Sun, 6 planar sourcebooks (Manual of Planes, Astral Sea, Elemental Chaos, Shadowfell, Heroes of Shadow, Demonomicon), a Dark Sun adventure, an Eberron adventure, a Forgotten Realms adventure, twelve Nentir Vale adventures, an Underdark sourcebook, two Draconomicons, an Undead sourcebook, two gear sourcebooks, a mini-adventures sourcebooks, two new Player's Handbooks, and class expansions for every class released up to that point! Not counting the stuff in Dragon & Dungeon Magazine - I know some Ravenloft purists will sneer down their noses at it, but at least we got new domains of dread instead of yet another remake of the original Ravenloft adventure module!
It's not that Faerun is a bad setting, because, really, it's not. The Forgotten Realms is the most popular and iconic established D&D setting for a reason, and it's a definitive example of the D&D style of high fantasy.
But, come on, seriously, give us a break! We're sick of it! Steak isn't a bad thing, but eating it for breakfast, lunch and dinner, seven days a week, for three years is a little too much!
That's why people are bashing the Forgotten Realms: because it's overplayed at this point. We want something new. We want a Sigil Adventurer's Guide, or a Manual of the Planes at least, for more cosmological gameplay options! We want a Khorvaire Player's Guide, so we can go back to Eberron instead of the Sword Coast! We want a Starfarer's Manual, to bring Spelljammer back to life!
We're tired of the same old dungeoning crawling stories. They're the staple of D&D, but they're well and truly played out at this point. Until Faerun gets a rest, people aren't going to stop kicking it in the ribs.