The official explanation : the rotation arrangement meant that the elders would always be learning and never become proficient in a given area. On the other hand, keeping each in his designated place makes for an experienced elder in each position. Much more efficient! That seems to make sense doesn’t it?
The real explanation : the Society quickly realized with their little experiment in self-government that it was imperative to keep a tight grip and maintain control of each elder body and congregation. How ? Simple ! They appoint the director, the big cheese, the main man, the presiding pepper. Enthrone a "Tower Bower". And if he stops bowing, they replace him rapido!
And that’s why the C.O’s are still visiting. Gotta monitor quality of the local’s "Tower Bowing" and check up on the titled "Tower Bower".