Additionally, dies for stamping cranks cost a hell of a lot. Over a million bucks. So if you are using the same dies (modified) for track and road, such as would be possible for 170 cranks, you could distribute that die cost over more cranks, meaning it would cost less to manufacture. In the case of 165 cranks, they need smaller dies for a smaller run, meaning the tooling cost is higher per crankset.