Hello! The reason is because to import a save into a new game, the new game has to include exactly the same variables as the old one. But while creating the new one, an author might need to tweak the variables in the old one which would break a game saved at that stage. (This happened to me when I was making my own sequel recently - if saves had been made when Crème de la Crème was first released, they would be broken by the time Royal Affairs came out.)
This is a good summary:
It’s likely that saving Book 3 progress will be available when Book 4 is at least halfway completed.