thanks for the reply...why was it to be imparted only by oral tradition?
If I had to hazard a guess, I'd say it is partially a remnant of times past, during which "secrets of the ryu" were not spoken of openly, and certainly not written down...and partially just the practical difficulties of imparting that kind of knowledge. Please understand that Aikido is certainly not a koryu, but it does carry some themes from that style of martial art, especially in it's early days, when the connection with Daito ryu was very strong.
I think you are going to find a lot of things in aikido that are either left kind of fuzzy, or even sometimes seemingly flat out contradictory. People either find ways to reconcile these things for themselves (or just live with the contradictions), or they lean on others to do it for them. I recommend doing it for yourself. With a certain amount of learning to live with the contradictions.