Where are you finding evidence to support your claim there is a delay at all? I know you want them to announce more earlier, and I know you have a theory that failure to announce earlier is somehow damaging to their marketing efforts (which I and some others disagree with), but where is the hard evidence that there was some deadline that they missed?
There's enough evidence to make an educated deduction.
1) D&D has traditionally released at GenCon with 3.0, 3.5, 4e, and Essentials premiering then (or immediately prior).
2) WotC did not confirm"release date" leaked by B&N.
3) WotC did not announcing a release date at Winter Fantasy, GAMA, or Pax East
4) The GenCon crunch for publishing companies is the first couple weeks of April. According to the Paizo editor-in-chief, for them it was the 14th to 18th this year, when they *had* to have books finished to be ready for the Convention.
5) At Pax East, April 10-13, Perkins said D&D5 was 97% done.
This strongly implies WotC was not certain the game would be ready for the desired release of GenCon, prior to this week. If anything delayed the book by even a couple days, such as a last minute rules error, the book wouldn't be out for the Con.
Which meant they were cutting it *really* close.
It's highly unlikely WotC would want to wait until the very last minute to have the game finished, so this implies delays. Repeated delays.
At this point, WotC knows for certain if they will make GenCon or not.
If it is GenCon, they can release that information now. If they can't make GenCon, they'll know it will be out shortly after, since they would have *just* missed the deadline. Likely September as they release at the end of the month. So it's safe to release that later date.
Which is also why they were suddenly hyping Tyranny of Dragons. They weren't certain D&D5 would make the cut-off date but had to do something at pre-scheduled panels and something to get gamers excited for their GenCon offerings.