I would guess there are a few factors involved. One, with two books announced and on the way, they will wait until at least one of them is out. That brings us close to August. Two, they just announced Icewind Dale, so will let that settle in. Three, what we do know is that there is a book that includes player customization; given the recent concerns, they're almost certainly making sure that whatever book comes out, incorporates these concerns to some degree. Presumably they were doing that already, and are working with at least one consultant, but they might also be sticking their finger to the wind and seeing how the community responds to their announcement. Not sure if that is at all related to announcing what the book is, though.
Anyhow, I would guess that we'll get trickles and rumors over the next month or so, but the official anouncement won't come until August, but probably before Icewind Dale comes out. Part of their marketing strategy seems to be stringing us along in terms of anticipation (and I don't mean this in a pejorative way). So we have:
...Icewind Dale is announced...
July 21: Theros
...digestion of Theros/rising anticipation for Icewind Dale...
...Theros is metabolized, anticipation on ID wanes...
...new book is anounced...
Sept 21: Icewind Dale
...digestion of ID/rising anticipation fo next book...
Nov XX: New book
It is kind of like overlapping waves...WotC wants to make sure at least one book is "cresting" in terms of anticipation, and probably only one at a time.