People say this all the time; "An FR Book would sell like hotcakes."
And yet, the folks at WotC only published the SCAG, and haven't published any FR setting book since, despite the wails that the SCAG is wholly inadequate.
Unless there's some actual data supporting this, it's really just a theory. If it was true, we'd likely have gotten it already.
Looking at Amazon sales rankings right now, the SCAG is only just ahead of Saltmarsh and Tales from the Yawning Portal (both Greyhakw books).
Yes, it is a theory--or hypothesis, really. What else could it be?
But my reasoning is this:
For a large number of current D&D players, the Realms is all they know, and the books have--for the most part--only explored the Sword Coast. I'm guessing that a sizable portion of these folks would be interesting in seeing the world more fully detailed in a modernized way.
The FR is still the most popular and used setting in D&D history, probably far ahead of everything else (EN World isn't an accurate microcosm as it skews much older than the current fan-base, and thus older, classic settings seem far more popular than they are with the general fan-base. This may be especially true of Greyhawk, which hasn't had a dedicated setting book published since 2000 (
Living Greyhawk Gazetteer).
But I agree that it is somewhat surprising that we haven't seen a full FR treatment yet. But that might be for reasons that we don't know: Maybe it was pushed back, or maybe they wanted to focus on expanding into other worlds, or maybe they wanted to wait and see.
I mean, it is only within the last few years that it has been made clear just how successful 5E is. While it was a success from the beginning, if I remember correctly the fan-base and sales didn't really start to spike until 2017-18ish, which is reflected in the increased publishing schedule from 2017 to '19.
My sense is that, for the first few years (2014-17), WotC was playing it safe and spartan with their publishing schedule, feeling things out--specifically how well it would do. It was clear early on that D&D was well-received, but it wasn't clear until about 2018, even 2019, that it was a smashing success. So it might be that they started to think differently about the future around then, including whether or not to do a fuller treatment of the Realms. So let's say they settled on doing a Realms book in 2019; the earliest we would have seen it is probably 2021-22. But by that point they had other ideas they were excited about, and wanted to publish first.
2024 also makes sense from an anniversary extravaganza POV: not only revised rulebooks, but other products, including a deluxe version of the game's most popular setting.