I am currently reviewing for my exam and a questions comes to my mind when I read an example about a simple key exchange protocol. In my textbook, the author comes up with an example about a simple key exchange which is the same as in the below image. The Needham-Schroeder, Denning-Sacco protocols are both based on this simple protocol.
My question is why C does not send session key directly to B? In my opinion, the reason is to avoid DoS attack to C. But is it the only reason?