Day/night cycles are already an incredible workload for just one, somewhat big, map. Considering the fact that this game is going to be absolutely huge, with a lot of different places to visit, implementing day/night cycles is a whole lot of work for something that's not really important to the gameplay.
Many huge games have D/N cycle.
Immersion is something that should not be dismissed in a RPG. I think this is where Larian needs to improve.