To my understanding, FR has been far and away the most popular kitchensink fantasy setting since it was published in 1e though, rapidly outpacing its kitchensink "competitors" Greyhawk, Blackmoor, Mystara, and Dragonlance. So while Greyhawk was chosen in 3e to be the core setting because of its legacy on the game, Forgotten Realms has gotten by on popularity first and foremost.
There's a reason they chose it as the core setting of 5e. I don't like it and prefer my homebrew setting and Nerath to it. But we're the minority, to my understanding.