More dev time needed
When Batman: Arkham Knight was announced back in March, they said we could expect it this year. You know how that goes. Now it's coming in 2015.
Rocksteady showed off Arkham Knight at a pre-E3 event a few weeks back. They opened their presentation with the news that they want to extend the development time of their game to ensure that they are "delivering the awesome level of quality that Batman fans and gamers expect."
So you'll have a bit more of a wait for the conclusion of this trilogy.