Why would you want that when:
If the DMsGuild is so terrible and full of content that nobody really uses, why do you care if Greyhawk is on there or not?Plus... having seen what people are doing with Ravenloft and Eberron, there's not a lot of setting expansion going on. How is the setting going to be supported by being on the DMsGuild more than it is right now? What do you need to play Greyhawk in 5e that you don't already have?
Yes, the dmsguild is rather terrible. I admit that. It's rather close to dandwiki with some of the content people put on there. But there have been some really good gems. Has been that way since they opened it up in 2016 (I put my first product up on there nearly 3 years ago to the date). Not many people I come across use the fan-made stuff. Opening more settings would give the chance of some gems popping up based in those settings.
Also, what do you need to play Forgotten Realms in 5e that wasn't already available from the past 30-40 years? Why focus on Greyhawk, and not mention one of the other settings. Greyhawk and FR both have basically the same races. But why not open up Ansalon and Dragonlance? There are races, classes, subclasses, monsters, and other things rather unique to that setting that are not available using the generic Forgotten Realms products. What about Spelljammer? What about Dark Sun? I see so many people pining for that setting (I don't see the appeal, but apparently a large number love it).
[MENTION=7006]DEFCON 1[/MENTION] : 5 years ago, they just barely ended the playtest, and wouldn't truly release 5e for another 6ish months. But yes, not long after release, people were clamoring for a non-adventure 5e book. It took just over 2 years for them to release something that wasn't an adventure: Volo's Guide to Monsters. It gave DMs new monsters, and players a few new racial options. And people loved it. The next year they released the first fully player-oriented book: Xanathar's Guide to Everything. And people ate that book up. A year before Volo's, they released the forgettable and disappointing Sword Coast Adventurer's Guide.
Now, thanks to Curse of Strahd, people are clamoring for them to release something for the other settings. And when CoS was released, and opened on the dmsguild, it was flooded with Ravenloft and horror-centric products, and it dwindled when Yawning Portal was released. Then came generic stuff, and when ToA was released, it was flooded with jungle-themed products. Release a Dragonlance adventure (hell, even a remake of the War of the Lance adventures would be alright with me) and it would be flooded with Dragonlance-centric items. Those who want to create products for Mystara may be aiming for a super small slice, but open Mystara up on the DMsguild and you will be flooded with Mystara products for at least a month, possibly two. Hell, even Birthright would get a huge flood of fan made products for a month, and I am sure that was the setting with the smallest fanbase back in the 90s.