WotC is in a Catch-22, setting wise.
Lets look at WotC's stable for a moment. If we focus on the big-draws, we have Forgotten Realms, Greyhawk, DragonLance, Ravenloft, Planescape, Dark Sun, and Eberron. They're the ones with the largest fanbases and most draw, and every one of them was printed in at least two editions or more. Now, let's do some analysis on them...
Forgotten Realms and Greyhawk are, for lack of a better term, big default dumping grounds where everything in D&D ends up living. There are certainly some differences in tone (high fantasy vs sword and sorcery) and completeness (fully detailed vs. lightly sketched) but in the end of the day, there is little GH offers over FR in terms of being "generic fantasy" other than those two elements and a handful of famous names and adventures (ToH, Vecna, etc) and FR has the advantage of pre-made synergy (its got book lines and video games) and lastly, for reasons uncontrollable to WotC, its creator (Greenwood) is alive and on good terms with WotC; Gary has sadly left us and had washed his hands of GH years ago.
Eberron is fairly kitchen-sink, but its Kitchen-Sink with a Twist. It's a better alternative to FR in terms of fantasy, because it offers a slightly more nontraditional view of fantasy, and its not one that invalidates the PHB or MM in doing so. Moreso then GH, there is a clear separation of style from FR, and its ideas on magi-tech are innovative. However, for Eberron to work you need to buy into the Twist (the magic-tech world) and if you don't it leaves you with little else.
Dragonlance is probably a better novel line than setting. The problem is it demands significant changes from the "default" world (no orcs, no halflings, no lycanthropes) and doesn't offer much back that can't be replicated elsewhere. As Tyranny of Dragons showed, you don't need Krynn to make a sweeping dragon-war epic, so the world ends up falling into a "same, but different" area where its close enough to generic to not stand out, too far away from generic to run without additional materials. I don't know how you solve this problem.
Dark Sun has the benefit of having recently seen a revival in 4e, and its very far removed from the generic. That said, its very "love it or hate it" as a world and requires some major rule additions to work. I don't know if the call for "S&S survivalist power fantasy" is that big these days.
Ravenloft, for all my love, is another setting that the best of it (haunted adventures, Strahd and his Castle, and horror) doesn't need its own setting, especially one so spliced together as the RL campaign world is. Like Dragonlance, its best parts can be mined out safely.
Planescape was only ever tangentially a setting. The Planes have been a part of D&D since the beginning, and Sigil has seen reprint in nearly every editions in some format. All later planar material was missing was the cant and factions and those are both very "loved or hated". So again, you can safely extract the best from it (such as its races and Sigil itself) without needing the full setting again.
Now, if I'm WotC and I see this, I see that Realms is probably the safest bet; its generic, doesn't upset the core, and has brand synergy baked in. Eberron is the next best because while it has a Twist, it doesn't need extensive work to get in moving and it can still host a variety of adventures and such. Dark Sun is on the fence; unless there is a giant demand for it, I don't see it selling well. Greyhawk the same, it doesn't offer much that can't be done elsewhere. Dragonlance, Ravenloft, and Planescape can be mined for ideas and stuff, but I don't see any of them being giant enough sellers to warrant the investment.
Takeaway: If the goal is to sell 50-100,000 copies, few of those settings are going to cut it. Definitely Realms, probably Eberron, maybe Dark Sun. The rest can be covered in other ways: a generic horror supplement or Castle Ravenloft AP will sell more copies than a Ravenloft Campaign Setting; a Planar AP or Planar Adventures book will do better than a Planescape book; a new Tomb/Giants/EE AP is a safer bet than another Greyhawk book, etc. In that regard, I can't fault them for picking ToD over a new Dragonlance book or putting the EE in the Realms rather than GH. Simply put, they can't make enough money supporting more than 2-3 settings at most, but they can make more money if those "settings" are boiled down and their best is used generically.