Development for encounters for a 12-13 subtier should take at least as much time as development for 7-8 and 10-11. Development for 14-15 will take as much time again, at least, based on the increased complexity of interactions to make GOOD story.
The other problem is that the appropriate framework of story to put those adventurers into are fundamentally different than the framework of story in which you put a subtier 1-2 table. They face different challenges with different mechanisms of resolution because of their vastly different capabilities.
Trying to fit both into the same event's wordcount can be very narrative-bending. Some of the better-received Living Greyhawk BIs were hundreds of pages to accommodate the need to have encounters that made sense across different APL bands - APLs 2 through 16, by even numbers, in their case. The writing and development time when doing this in a professional manner as Paizo is doing is just not feasible.
Hell, I was concerned we might not see ANY GenCon special this year due to time budgeting issues. Knowing that we traded 5-Ex being new content for 5-Conclusion and 6-00 both being things is great, and I can't wait to see what the factions have turned their attention to this year.