The only had the code to arkham city, and those assets
Arkham Knight is an entirely different game with different assets, all they could of did with that old stuff was make a better looking Arkham Origins
Rockysteady is working on suicide squad game, they had no choice but to make a game from scratch
Even in Origins you could see a difference in the gameplay, it was a sluggish, and the level design was off, Batman couldn't glide has a high almost like an invincible ceiling and also the grapple hook couldn't grapple on things he should've, that's why they had to put fast travel in the game because moving around the map is tedious
in Arkham Knight the map is 3 times as big, no fast travel, because it's not needed, Batman can quickly move around the map even without the batmobile