I feel like Larian said that it'd be a huge amount of work to implement a day/night cycle where all NPCs and enemies have different routines/dialogue/etc during day vs night.
It is much simpler to implement a day/night cycle that is mainly cosmetic. Except for perhaps -some NPCs simply aren't available (they're sleeping). Removing NPCs from the map is much easier than given them a whole new routine -stealth is easier. This can be a global modifier where enemies have shorter vision cones or disadvantage on perception checks. Or just properly account for creature's level of darkvision, which is already in the game -add a few night-only enemies here and there. Wolves, vampires (at higher levels), maybe more goblin guards to be on alert against nighttime attacks
