Some people got it and some people don't. It's like an art class- you can take 50 random people who aren't artists teach them a few basics and watch the "true" artists come out of the woodwork. I don't know what it is but some people are just more gifted than others. Some just work harder and are blessed with developing a gift.
I have been studying mechanics for years and have come to the conclusion that for some unknown reason some people can just throw harder than others even with the same exact looking mechanics. Some researchers now are saying that it has to do with timing, which i largely agree with. The problem though is knowing what actually constitutes great timing. We can watch slo-mo replays of the best and fastest pitchers and realize there is not much difference with everyday average Joe down at the local schoolyard sandlot.
I do know that correct timing and muscle- the right kinds of it are more factors than anything else. Now if we could just figure out which muscles to train and how to orchestrate it all into a perfectly timed event and we would be composing great music!
If ya got it ya got it, and if you don't got it, you better work a little harder than everyone else and pray that the baseball gods bless you with a gift.