The problem with settings is, by definition, they exist to segregate the market.
The biggest problem is that a DM might need or want one, maybe two setting book tops. Most DMs never use more than single setting and never more than one at a time, so unless your DM is big into world-hopping, most of those other books are not going to be on that DM's radar. Myself as an example, I'd have bought a Faerun, and Eberron guide but I'd have zero interest in an Athas or Krynn guide. They aren't even worth it for additional character options as a DM who is running Dark Sun has little need of anything in an Eberron guide, and vice versa. So each guide limits who is going to buy it, and the less "generic" the setting is, the small the niche market is.
Further, if we assume WotC would support a setting beyond a single guide, there is even further diminishing returns. An adventure set on Athas is literally useless to me as a Faerun DM. Athas is so far removed from the traditional D&D tropes that it couldn't even be used as a "generic desert adventure". An Athasian AP is a product of interest to A.) DMs who B.) Want an adventure path C.) set on Athas. That's a rather limiting market when WotC could get much better mileage out of a Destert of Desolation AP loosely set in Raurin and convertible to Eberron, Greyhawk, or Mystara easily.
Paizo realized this and wisely put all its settings on the same world, so Golarion is the meta-setting for everything from Eldritch Horror to Sword & Sorcery and back (and it eliminates having to create constant variants on the core game assumptions, which range from renaming subraces of elves to rewriting the PHB classes) but D&D's ship on that sailed. I still think a primer book on a multitude of settings is a far better use of resources than 6-7 SCAG books that compete with each other.