Agree that is really a complicated set of factors involved. But if there one simplified word I think of to describe what can be improved upon to gain velocity, which is "leverage". And you need strength too to be able to use the leverages along the bio-mechanical chain well. Most HS pitchers, though they might have plenty of strength (even fast twitch muscles), just don't use enough leverage to get the acceleration to the higher velocities. Yes, it's mechanics that addresses leverage. Strength is very important for endurance and probably a main reason pitchers work so much on their strength not only for the upper body but also for their lower body.