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
+1. Also to balance the benefits off the night (stealing, trespassing, + to stealth checks) it could be nice - and perhaps not too hard - to implement a kind of night-watch/guard system aka flaming fists, angry druid-novices on guard, town-watch which spawns/guards/patrols the area while the others are 'sleeping' or away.
They could have an escalating system where they first warn, and eventually attack the party or PC depending on the crime they commit. Given the current pandemic, for instance, it would be sensible for the druid grove to enforce a nighttime curfew to avoid an 'enemy within' to open the gates. Hence, depending on what you do at night (sneaking around vs. getting caught red handed while stealing) you get a warning and option to talk yourself out of it, while repeated offenses and crimes result in fines, or guards aggro...
