I know many will disagree with me .... but sometimes WoTC reminds me of the Eric Cartman-type child who has the cool toys but won't allow Kyle to touch them because he can. I'm not saying I want the Realms completely wiped out of publication .... but how about a 1 and done campaign book in the vein of Sword Coast Adventurer's Guide? Do one (and only 1) Greyhawk book like that. Same page count, same price point, I'll be more than happy to buy the campaign map and City of Greyhawk map from Mike Schley, and then continue to offer setting suggestions in their two main adventure books they do each year. Same thing with Planescape. Instead of saying everyone from all over the multiverse comes to The Yawning Portal ..... they could have done a 1 and done book on Sigil and the planes. Like I said about the Greyhawk suggestion, let me buy the Sigil map from Schley and do the same page count and price point as SCAG.
Lets look at the Opportunity Cost of a GHAG.
1. Right now, WotC's model is 2 APs + 1 Sourcebook. A GHAG would fill the role of the single sourcebook. Presumably, WotC would want to release an AP that coincides with the GHAG. So 2/3rds of that year's* releases would have to be related to Greyhawk related books for it to be cost-viable.
2. The Adventurer's League is tightly tied to the current storyline of any given cycle, but it is heavily Realms-centric. A GHAG and GHAP wouldn't be usable to AL. They would be stuck with a terrible dilemma of allowing the existing FR-based AL characters into GH to play the GHAP and use the GHAG or disallowing any previous AL characters in the GHAP, meaning anyone wanting to play that season has to start with brand-new level 1 PCs. (It was a headache when AL tried this with Curse of Strahd, btw). They could, of course, ignore the GHAG/GHAP and do there own Realm-centered thing, but since AL is primarily a marketing tool for WotC, I don't see how that helps sell product.
3. Right Now, the Neverwinter MMO takes stuff from the current Storyline to incorporate into the game. A GHAP/GHAG would be unusable for new content. Like the AL, they'd have to convert it to the Realms anyway OR ignore it do there own thing.
4. Right now, a DM who is running the Realms has no use for a GHAG, and a future GH DM would have no use for the SCAG. Why would WotC put out a book that competes with another book it sells when it could put out a book that compliments it (like how XGtE and VGtM compliment SCAG).
Believe me, I thought I'd see an Eberron book by now myself, but I'm beginning to think that a full-hardcover book dedicated solely to a non-Realms setting alone is too much sacrifice for too little gain (IE the number of people who would/could not use it isn't less than the number of new people who it would attract). The more niche the setting (Dark Sun, Dragonlance, Eberron) the less people will buy it. Now, if WotC was to introduce a 4th book into the mix (say sometime in early summer) all bets are off, but for right now, when you only make 3 books a year, your best bet it to try to not limit who would buy it.
* The sourcebook usually comes out in Nov, APs in March and Sept. So either the Sept module is GHAP and there is a two-month gap before the setting book comes out OR the setting book comes out and then the spring AP is dedicated to GHAP, which leaves a 4 month gap of a setting with no AP to run with it. Again, WotC would have to change its release schedule (and thus, all its project deadlines) to make it work.