If they had millions more to spend, they could spend months working on the day/night stuff, but we don't know how much the publishers are funding them.
There is no publisher; Larian is self funding much of the development, and Kickstarter added on to what could be done.
From the
D:OS 2 Kickstarter FAQ:
No, we have no plans to introduce a day and night cycle. What we're trying to do at the narrative & scripting level is really complicated, so to even contemplate adding yet another extra dimension of complexity to that gives us a headache. The impact of the player origins combined with a higher level of freedom than the already insane amount of freedom you had in Divinity: Original Sin, added to the options required for multiplayer mode already means that our development is going to be hugely complex. Until we tame all of that, we won't add an extra layer. Perhaps for another game, but definitely not for this one.
