reason for such...
If I understand correctly what I have heard, titanium reacts with nitrogen readily to produce titanium nitride, commonly used in ceramics. A good friend of mine said that he has observed titanium barrels when playing in lower light conditions actually spark when fired, a visible sign of an oxidation-reduction reaction taking place such as occurs when any metal combines with oxygen or other non-metallic elements, in this case nitrogen. If this is indeed true, then the reaction would somewhat wear away at the inside of the barrel, causing the surface to be uneven hence not smooth hence not as accurate as a non-corroded surface such as aluminum, nickel, or stainless steel.
Thus, what may start off as a smooth inside of the barrel which gives good accuracy may become pitted and rough (if only barely perceptible, but non-smooth none the less) and thus not as accurate as originally intended.
Just a little food for thought.