Originally posted by eyrei
Only the Duke of Marlborough or the Duck of Flanders would truly be able to answer that. I imagine it had to do with cohesiveness in the beginning of the game. While the country is still small, it is quite possible and more efficient for one person to control all of the domestic department responsibilities as well as the build queues. If you had a domestic leader and a governor of all of the cities, who disagreed on everything, the expansion of the empire would flounder. It also allows the domestic leader to lead by example for the other governors as the game progresses. Further, as the domestic department is in charge of wonder building, it gives the department choice cities to build wonders.