If you had to ask me, I can't really give a scientific answer. I can only come up with a simple one like how in-tune you are with your body. Besides, sprinting is a skill - the more you train for it, the faster you are. Technique is just as well important. You might think it's just running or what not, but there're multiple phases in sprinting and the activation of your leg muscles to stimulate all that explosive action, not forgetting those that complement foot striking. Psychomotor is likely a factor as well. Maintaining dorsiflexion at the right timing, et cetera. At our level, sprinting is more or less a 'talent' or something that you train for.
Speed in general, however, is very dependent on a number of things. A strike from a Chinese Kung-fu practitioner reaches probably 400m/s - as fast as a cobra's bite. Now this example here is a perfect one for those who believe that mass would only slow things down. On the other hand though, look at boxers and muay thai practitioners in MMA circles - muscles ARE there, and it don't slow them down either! But of course when you compare weight classes there's a significant difference, though that difference happens only in elite levels of sports.
If I really had to say anything about speed, it'd be largely biomechanics and technique with regards to each and every sport. Can a bodybuilder sprint as fast as a international athlete? Probably not, but there're those who can sprint just as fast as anyone who does sprinting for leisurely training and fitness endeavours. Genetics count a lot as well!
Here's a final example of my favourite classic bodybuilder - Franco Columbu still practiced a little bit of boxing even in his bodybuilding prime, but if you ever came across any videos of that, you'd notice that he isn't one bit slow.
