Dragonlance was an attempt to create a dragon-centric setting, as none of the other current ones at that time had dragons front and center.
Forgotten Realms was deemed necessary for them to sideline Greyhawk and get out of paying any more royalties to Gygax than they had to. They kept publishing the already– or nearly–done Greyhawk stuff to fulfill some contractual obligations (and to insulate themselves from any Gygax lawsuits of "you guys are just doing it to keep me from getting paid")
They didn't want to port over Mystara as they were afraid that too many AD&D players would see it as the "basic" setting and not go for it.
Ravenloft was just an outgrowth of the gothic horror idea that was never really intended to grow like that; it wasn't planned beyond that first module.
Dark Sun & Birthright were trying to experiment with some new concepts, and Spelljammer was a ham-handed attempt to knit them together.
Eberron was created from a contest, where WotC was looking for a new world to make the centerpiece of the 3e launch. They specifically wanted something different than the "usual" but still could contain all the canonical races / classes (which really limited the "different"). So they basically got a reskinned Earthdawn.
But as MD noted, FR got more/better novel support, especially since they assigned a bunch of their better writers to that world, and that really helped it blow up.