Fourth edition was a (relative) failure. Wizards saw their flagship game (no not Magic, the other one) beaten in sales by an iterated version of its very own previous edition (Paizo's Pathfinder). Paizo stole the crown from Wizards as King of the RPG. They improved the parts that fans wanted improved, left the rest alone and put it all in a professional and well designed world. The best developers fled from Wizards en masse, some working for Paizo, many starting their own operations publishing compatible material under the Open Game License (gaming's version of the GPL).
D&D Next is a lot more like third edition than it is like fourth. Wizards wants their crown back. Time will tell if they're just going to be a pale imitator in their own field or if they'll actually pull an innovative iteration of D&D out of this.