For one it does not allow for multiplayer option as it can't be implemented well in RTwP.
Second they would need to cut back on so many features that it would not even resemble a 5e play style.
Three they would have to redesign the entire game & AI to be more simple & easy that it would give a terrible turn based mode like the crappy & suboptimal turn base mode in Kingmaker.
All this has already been tried in DOS2 by RTwP fanboys that it failed miserably. It is easy to turn a RTwP game into turn based but not the other way around as the AI in DOS2 was too harsh & fast for RTwP. Much like Final Fantasy changed play styles from the current & first iterations let this one be more true to d&d as RTwP was unfaithful to ad&d in BG 1&2.