I am so freaking tired of FR. I've been playing in it on and off since the late '80s. Even back then it was too complex for me to know all of the parts without reading all of the novels and such. Trying to play a bard without putting in homework to read bunches of novels of wildly varying quality, ugh.
And the NPCs. Oh my god. Super powerful with divine backing that are willing to not do somethign so the players can adventure.
I've never RUN the Realms because I can't run a world that the players know significantly more about than I do. "Oh, that sigil means that the Red Wizards are involved, and since we're near myth drannor obviously they are looking for elven 10th level magic." "Umm, no, I just made up a sigil." or "Oh, we're near Waterdeep, have our characters heard rumors about the well in the inn or other means to travel to Undermountain or Skullport?" "Whoswhat to where?"
It's been patched and plastered so many times to make it Fresh(tm) and New(tm).
I actively dislike the FR. Seeing FR on a product is a turn off. (Luckily, 5e is such a great edition it doesn't dilute that.)
Try producing another setting early in release cycle for three editions of D&D and then compare the numbers. Because as Morris says, they are assuming correlation is causality and people are playing FR because they prefer it, and not even evaluating that only having one officially supported campaign may make have something to do with why it was picked.
Actually, I've got a possible test. Let's ask a bunch of Pathfinder players if they run in the Forgotten Realms. There is plenty of 3.x FR material that's easily compatible, and I'd hazard to say that many pathfinder players are either former D&D players or have played games that former D&D players have run. If FR is actually a popular setting, I would expect that we would see a significant number of PF players who have kept it.
I played Golarion when I played Pathfinder because all the APs are set in Golarion. Makes it easier to measure travel and have a sense of location.
I likely wouldn't play Eberron. I'm not a fan of Steampunk. I much prefer traditional D&D fantasy worlds based on a pseudo-Medieval, Tolkien inspired world that is mostly low tech with high magic. If D&D changed worlds and had a similar world, I'd probably play that because I enjoy having a sense of location in the game.