While allowing the party to be split up, with some in real time and some in turn based combat, does cause issues with syncing a day/night cycle, that isn't really critical. The main problem is just the added complexity for doing a day/night cycle properly, on top of already complex scripting and narrative. With level design every NPC needs a place to go at night, those places need contents, etc.
And these were insurmontable difficulties?
Technically, you could make time go by despite being locked in turn-based combat. Would it be immersion-breaking to anyone? I don't know.
