Toben the Many said:
As far as "single book settings"...WotC has released those and continue to do so. Frostburn, Sandstorm, and Stormwrack aren't just environment books, they're setting books as well. They just didn't limit their setting to a singular place so that players didn't feel locked into their storyline. But the elements are there. There's pratically a mini-campaign in each of those sourcebooks.
Heroes of Battle is really a setting book in a different guise. It's created for people who want to run a war-time campaign. Heroes of Horror will be the same - a guide for people who want a Horror D&D game. The difference is that WotC doesn't want to nail down any of these products to any particular setting so that they can appeal to a wider audience.
Actually, I would say that you are slightly off the mark. Settings carry some degree of over-plot with them : Greyhawk had the fiendish forces scheming to take over the Prime; Forgotten Realms has the "recovery of the lost lore of fallen empires"; Eberron has its own more detailed over-plot (or so it sounds like to one who pays only slight attention).
What I see WotC doing is going less for whole settings, and more for very focused but fairly complete treatments of specific areas that were not covered in the core. The Core books cover Temperate land terrains (forests, hills, mountains, marshes, etc.) pretty well. The Environment series addresses the ones that were listed, but not fully detailed, in the Core: Hot, Cold, and Aquatic (for all climates).
Similarly, the Genre series seems to be focused on certain genres. With only Heroes of Battle out so far, it is impossible to say I see any pattern yet, but if Heroes of Horror (and any others yet to come) follow the same design, they will be a "bolt-on" toolkit to enable that genre with the Core rules.
In short, instead of publishing a specific "Wars against Aberrations in a Cold land" setting, they published three toolbooks for a DM to make the "perfect personal version" of such a setting: Heroes of Battle, Lords of Madness, and Frostburn. For DMs who would prefer to face hidden hordes of Undead under a scorching sun, use Libris Mortis with Sandstorm and the forthcoming Heroes of Horror.
Trivia: How many recognized some of the touchstone sites in Sandstorm as being locations in the old Desert of Desolation module series (I3-I5) ?