One of the less known reasons for the length of a mast is torsional flexibility.
I touched on the two beats per rev vibration for two bladed rotor systems on another thread. Torsional flexibility helps to reduce the effect. In that way, the mast is continually flexing about its own axis as drag increase and decreases across the disc. I hope that this does not scare the readers...! If it were made shorter it would be less flexible and suffer much more stress.
There is a good explanation of torsional flex. in Shawn Coyle's book.