I've never played top level sport like most of us here I guess. What ever separates those that can from those who can't quite is a mystery to me.
I've watched a lot of them up close for a long time and it is a mystery to me as well.
If I had to make a hypothesis I would say 95% of it is your innate ability, whether physical or genetical make-up. Not just the stuff we can measure but the intangible ability to do physical things others can't, like catch the ball when others would drop it. I'd call that the Dustin Martin or Shane Warne factor.
Then I reckon maybe 3% is your mental capacity in all sorts of ways, desire, motivation, capacity to handle fear, discpline and just the want to do it.
The other 2% is a mix of the people in your life, family, friends, team mates, coaches etc and of course, luck.
One thing I do know for absolute certain is if anyone tells you they can pick those who will from those who won't, before they actually do or don't, is a liar. Which is why drafting is a lottery.