Update 11 - Confused

[Dark83]

The numbers DO NOT lie as no other world has been as popular sold as much merchandise or been played in as much by D&D players.

That does not necessarily have anything to do with the inherent quality of the setting.

Both Greyhawk and Forgotten Realms have always been generic fantasy settings. FR has more materials written about it because it's been active for at least twice as long as the next longest supported setting, not necessarily because it is better or worse. All of FR's major competitors (Dragonlance, Greyhawk) had problems with their mutual publisher - notably Gary Gygax was forced out of TSR in 1986, and FR was put into production in 1987 after a year-long legal battle over TSR and Greyhawk materials.

Greyhawk was the default D&D setting for a while, first published in 1980. The names Mordenkainen, Bigby, Melf, Otto, Drawmij, Tenser, Otiluke, Evard, and many famous others are from Greyhawk, and we see many of their spells in DDO today. After Gygax was forced out of the company, FR more or less became the default setting until WotC bought TSR and 3e switched back to Greyhawk for the default.

Dark Sun, Planescape, Ravenloft, Spelljammer, and so on tend to be more niche - not general-use settings. Dragonlance was started in 1982, but the creators also ended up having issues with TSR. When TSR threw their support behind FR after having issues with GH, Weis and Hickman felt DL was neglected by TSR and went to Bantam Books in 1988, not returning until 1996. We'll never see how DL may have done if both TSR and the creators hadn't more or less ignored it for almost a decade.

So if you're trying to convince people about the quality of something, you can discuss its merits, but don't tell an economist that sales is a measure of quality. McDonald's has epic sales, but that doesn't mean the quality of their food is comparable to food from any Michelin-rated 3 star restaurants. The same could be said of VHS vs Betamax, recent video games, television shows, movies, or the recent generation of Apple hardware. Economic success isn't necessarily linked to product quality.

P.S. Not that I have anything against FR. One of my longest running campaigns was set in FR (and is still the favorite amongst the group, even though I haven't been the DM for over half a decade). I was a stickler for canon, and even had assorted lists of things like wine and cheese appropriate for certain regions. Something which would be impossible to compile in any other (less published) setting.