Why was the historic Greyhawk Castle never published?

Greyhawk was mentioned in the OD&D rules, in the Greyhawk Supplement, and in stories by Gary in Dragon and other magazines. There was a lot of interest in it. However, neither the dungeon nor the city were ever published. Why? The reasons changed over time:

1. The dungeon was huge and was constantly expanded, revised and restructured [1]. There is no single Greyhawk Castle. The dungeon was a living project, where multiple groups played in a shared space and each group influenced what other groups would encounter[15]. Gary also modified the layout and monster populations between sessions, to keep things interesting for returning groups [1, example from play]. We may desire a single, absolute truth, but that never existed. This experience was hard to capture on paper as a pre-written module. 

In short the original upper and lower parts of Castle Greyhawk changed many times over the years they were in active use. #1628

Pray tell, what is it's original form? The first level I drew up, the 2nd, etc. Maybe all the 13 I had--but wait! those were altered by PC actions. then there was the second version with continual changes, and the third which is fairly static. [35]

Hundreds of different players with yet more PCs adventured in city and castle, blasted buildings, created constructions, wiped out walls, closed passages, created new ones, trashed monsters, brought in others, and who can say what else! [15]

Furthermore, there is only "original" Greyhawk Dungeons, that being the 13 or so maps I first did. Most players adventured in those when they were altered by previous PCs' actions, then by my additions of side levels, finally a whole re-design of the castle that I made and added some maps from Rob to flesh it out. The dungeons went through three or four incarnations to get where they are now... [35]

2. The campaign was a home game, Gary was making up levels over night [3, 15] to keep up with demand of his players, and levels consisted of the floor plan, and a one-page minimal key about monsters and treasure. All the detail was was made up "out of whole cloth" on the spot and in reaction to player activity, and only the sketchiest of records about this were made. We know from Blackmoor Castle that the raw, unedited notes are not sufficient to capture the experience of the dungeon, and so did Gary. They would not have made a good product for the vast majority of buyers, who were by then used to detailed adventure descriptions, and would likely have caused disappointment if published so. 

Do I have the original material written about above? Well, most, if not all of it. [...] As for the castle dungeons, of course! The main problem is that the lot of it is in typical DM form, quite suitable for the campaign's directors to manage when immersed in the action, but rather cryptic now after all these years. [15]

If the GM was familiar with my style, imaginative, and enjoyed winging play, the notes would serve well, as they did for Rob Kuntz. Otherwise, they would be disappointing at best. Imagine: "5-8 bugbears, guard with m s +2 damage, each 10-60 g; c trapdoor/trap with spikes in NW leads to niche with 1-2 random magic items." [6 #2761]

In the original campaign run in Lake Geneva, much of the refereeing was done "seat-of-the-pants" style, and encounter areas were not fleshed out much beyond quickly jotted notes. This, obviously is not conducive to the development of a published adventure run by and played by those who are not its creator: Thus, a derivative must be developed in a publishable format. [7]

Now you understand why the Castle Zagyg project is such a major design undertaking. If we handed over the binders containing the maps and the notes don't think even the ablest of DMs would feel empowered to direct adventures using the materials. ..unless that worthy was someone who had spent many hours playing with Rob and me as DM. [2]

3. Gary also was enjoying his home game, and did not want to give it up by publishing its secrets so all the players could read it. Even when he finally got around to start publishing it, he did not intend to give away those secrets.

The material from my GH castle-dungeons campaign didn't get generally published because I was playing it, and had no interest in trading off a perfectly good adventure setting for a few bucks. #486

Just don't expect the secrets of the Disappearing Jeweled Man and the Great Stone Face to ever be revealed in actuality :eek: I plan to provide a set of options for the DM that may or man not include the way those were dealt with originally. #596

Of course some possible explanation will be given in the "Zagig's Castle" work...if that ever comes into being. Of course I never planned to give the actual information I based things like you mention above for my campaign--at least not obviously. the very best part of mysteries is keeping them that. This can be done by offering multiple options for the Dm for the "Disappearing Jeweled Man," the instant evoker of greed in my group of PCs, and the "Great Stone Face," something as enigmatic as Stonehenge.

Of course even as I give a choice of explanations for those things I wil have to include new, unanswered riddles--or at lease highlight some previously unknown ones there in the mazes below the castle.

Life is full of unanswered questions, and I believe that the RPG would should be the same #1129

Gary originally also did not believe anyone would have interest in pre-made adventures. He expected GMs would design their own dungeons which was easy enough to do for him, instead of wasting time reading someone else's. In 1975 he wrote about how a GM should spend a week to set up his own campaign map, city, and dungeon levels before they could start play [3]. (Note that this does not match what he did, he just started with level one, and added things as he needed them [15]). TSR did not get into publishing modules or campaign settings of any kind before 1978. 

4. Writing up the entire castle in a form that was useful was a mammoth effort. Three of the "extra-dimentional" side levels, which could be run as standalone sites independent of the dungeon were published: Dungeonland, the Land Beyond the Magic Mirror, and Isle of the Ape. They constitute the only published two levels of it until Castle Zagyg, and each took an entire module. The core would have been 13 full modules he would have had to write mostly himself, as there were no detailed records for others to work from.

Publishing a revised version of the original castle ruins and dungeons from my campaign begin in 1972 has been considered, yes. Frankly, the project is so large, would consume so much time, that I can not consider it alone. So far I have had no success in reaching agreement with another person in regards to co-development of the material. Of course I require someone who actually played extensively in the campaign and knows how to design adventure modules. [6, #42]

5. After he was ousted from TSR, he lost the IP to the name Greyhawk, as well as the characters from his campaign that had made it into D&D, like Tenser. He also was sued by TSR when he tried to launch a fantasy role playing game. Anything too close to Greyhawk would have carried the risk of yet another costly lawsuit, even if it was unjustified. So he could not do it at that point, where he still would have had enought health, time and energy. 

6. Gary did not want to disappoint his fans. From what we know about the old dungeon, it was rather random and an historically accurate replication might have been a disappointment. By the time towards the end of his life he finally undertook the project to publish the dungeon, in the introductory notes he is defensive about its simple nature and lack of logic or ecology: 

The whys and hows of the monster population in the gloomy labyrinths were unimportant to all concerned. The sense of danger in mentally exploring a lightless maze filled with perilous traps and fell creatures of unimaginable sort was far more important [7]. 

He planned to create a dungeon of  "all the best features of the former levels" that could stand up to contemporary efforts. He did describe how he saw the challenge of transforming the minimal notes into a publishable adventure.

This should not be considered a replica of the original dungeon level as invented for game play during aurumn of 1972, because this dungeon-and this entire mega-dungeon adventure-has gone through many permutations over its three plus decades of play and development. It has been expanded, contracted, complicated, and simplified several times over by its primary architect. Its content has been affected by the game play of its original and subsequent participants; likewise, it has been modified for convention use and modified by convention use. Furthermore, Rob Kuntz added many of his own levels, sub-levels, and extra-planar excursions when he served as referee. The dungeon level presented here, and those that will follow, is inspired by and draws from the original work; the original maps, hand-written notes, and special encounter areas have been perused at length, and some of the original players have been interviewed extensively while drafting this newest, latest incarnation of the Mad Arch-Mage's infamous dungeon. [7]

Unfortunately, Rob dropped out of the project,and Gary sadly passed away after the first level was done. 

Even though he must have realized that he would not finish publish the work as he envisioned it, he was violently against publishing the original maps and materials. Someone on the forums did a poll that showed strong interest in them, and lobbied for it with him, and this is what he had to say:

How many of the actual audience of over three million active D&D players did that poll reach? It is absolutely meaningless in the marketplace. It merely indicates a few dozen people would possibly be willing spend more money than they should for shoddy copies of something. [35]

As I said, it is a silly idea, and those that think it is a good one are not considering the ramifications of the whole matter. There is no earthly use to most GMs for reproductions of sloppily hand-drawn maps and one-line encounter notes, other than to say thay have a copy. The heart of all the adventures in those dungeons was improvisation. Production of such copies will also spoil the opportunity for creating a truly usable dungeon complex based on the material broadcast thus. [35]

Material which is generally unusable without a great deal of alteration and additional text; a new fifth incarnation. As I say, I am not in the business of creating souvenirs but rather playable and otherwise useful entertainment products. I don't charge for autographs, and I don't have headshot photos to sell to fans either. [35]

I won't be creating souvenirs for sale anytime. Those persons that are into such things might want to buy them if items from my estate are auctioned off after my demise [35]

I think he underestimated the historical significance of those "shoddy copies". These were the original notes. Yes, improvisation brought them to life. But they still are much more significant than a new, polished dungeon inspiried by them, which would have been only be one amongst thousands. 

[References: see Greyhawk References]