I think their biggest problem is how to do time in MP, which is the same engine as SP, but allows independent control of each player's time stream. If one player is exploring at, say, 5 real-world mins per game hour, while a second player decides to go into TB mode, where time essentially stops, occasionally advancing by 6 game seconds in a much greater amount of real-world time, then you are in a mess very quickly.
They could make SP and MP modes different at fairly significant development cost, but I don;t think they want to. They could also make TB apply to all players in MP whenever it applies to any one player, but I don't think they want to do that either.
If someone here that wants D/N can suggest a good mechanism for MP, then maybe Larian mat reconsider.
Whilst I don't know the codding challenges related to the issue, I have to add that Neverwinter Nights did MP coupled with D/N cycles 20 years ago and BGEE adapted the original code to work with MP, so it is definitely possible to code it.
Yes, implementing D/N cycles isn't particularly complex, and has been done many times before. It's somewhat gone out of fashion with some developers, but I've not read anywhere what their reasoning is.
It might be due to player pressure, since some players dislike dealing with people moving ( having to search for them ) or finding shops closed when they want to use them etc.
It's also possible that there are technical reasons; for example, some high quality lighting solutions in games pre-calculate illumination for scenes, which means there cannot be dynamic, time-based global illumination. You could still do D/N flip similar to Athkatla in BG2, by simply pre-calculating 2 lighting solutions.
There would definitely be a cost attached to adding a D/N cycle, but only Larian know exactly what that would be, and they are not saying.