Neonchameleon
Legend
A couple thoughts that come to mind:
1. 5E was more popular than expected, and they've decided to offer a heavier publication schedule but aren't sure yet how it is all going to play out.
2. They're scrambling to figure out what to put out, how to finish what they're working on, etc. All hell has broken loose in Renton -- dogs and cats living together, mass hysteria!
3. They're playing it coy to build up anticipation, a sense of mystery, yada yada yada.
What say you?
Or some combination of the two.
You missed one obvious possibility.
4: 5e going forward is essentially D&D in mothballs. That sales numbers appear to have been lower than for any previous edition since the red box (counting 4e but not 4e Essentials) and ultimately it isn't terribly popular (both 3.0 and 4e made it onto the NYT bestseller lists). People bought the PHB because it was the new PHB - but most 4e fans stuck with 4e (and large numbers had stopped buying in response to Essentials) and most PF fans stuck with Pathfinder. The D&D team is smaller than it has been at any point in history, and the other two core books didn't sell remotely as well as the PHB.