Because WotC has explicitly said they are putting out new settings because they offer a new-to-this-edition playstyle. This was explicitly said in reference to Theros and Strixhaven, but it was stated as a general principles.
Now, my first D&D setting with the Greyhawk Folio, but is wishful thinking to say that the kind of adventures that take place on Oerth are, generally speaking, very different than those that take place in the Forgotten Realms, generally speaking. (Yes, we can definitely find some corner cases where each setting is unique -- ISLAND OF THE FLYING MONKEYS! -- but this is a general statement.)
WotC are also the beneficiaries of TSR losing money by putting out settings that competed with each other for the same handful of gamer dollars. They're not going to do it again.
The World of Greyhawk is great. I love the World of Greyhawk, but by WotC's definition, there's not enough in the setting to make it a distinct, non-competing setting when compared to the 800 lb. gorilla of the Forgotten Realms, which they have invested, relatively speaking, a lot of time and energy in.
So, what does have Greyhawk have to offer that might be commercially compelling for WotC? 50th anniversary nostalgia, especially if they can convince Gail Greyhawk to let them use some of the material she's sitting on.
But in 2022, 2023, 2025, it's a setting that competes with the Forgotten Realms. So a no go.
(That said, if it's not already unlocked on the DMs Guild, WotC ought to do so, ASAP.)