One of the main reasons why a day-night cycle is a problem to implement is the possibility of splitting the party, like I've said before.
In DOS1-2, and in BG3, it is possible for some characters to be in TB while others are not in combat but in RT exploration mode. Technically, time would be paused in TB, passing at 6 seconds per round. But at the same time many things can happen for other characters in RT, they can even travel the whole map and join in the fight, all in technically 6 seconds combat time. Time is highly abstracted there.
This becomes even more jarring and problematic with a day-night cycle. Technically, there should be no cycling in TB, since most fights last around a minute. But obviously time would need to pass in RT. So it would be possible that, while in TB time is fixed, a character in RT would spend like 2 full cycles (2 days). It would be possible that, while the fight in TB is during the day, a character in RT sees time pass and then joins the fight while his worldstate is at night. That would just be a mess, especially if mechanics are tied to the day-night cycle.
For example, say a character in RT where it is nightime joins a TB fight happening during the day. What happens? Does it suddenly become the day for all characters? If not, which visibility and sneak rules do you apply? Night guy attacks others as if it was night and day guy has day modifiers for visibility? Night guy is able to sneak better than day guy?
So given how time flows in Larian's games, because of the possibility of having characters both in TB and RT at the same time, adding a day-night cycle creates non-negligible problems. They'd have to redesign their systems entirely to take into account the cycle and time passing, and likely scrap the possibility of having characters both in TB and RT at the same time. So the question they have to ask themselves, are the benefits of implementing a day-night cycle worth the negatives and what has to be scrapped for it to work? It's a huge trade-off, not just a simple thing to add with little consequence like many here are saying.