On a purely rational level: D&D needs less settings, not more. Too much of D&D is fractured into settings that are 65% alike but 35% different enough that options don't port over well. Dragonlance elves aren't exactly the same as Greyhawk elves. Ebberon's planes aren't configured like Planescapes. Kender and warforged can't adventure together without world hopping shinanigans. The Scion of the Three subclass must be refluffed to work on Exandria, etc. You create these ghettos of design where people argue X doesn't belong in Y setting or you get overly generic fluff that doesn't tie things to the world because it has to work in any world.