If we're going to make the sports analogy (which I don't think applies to music 100%), it's not any one thing. It's hard work, practice, natural talent, how young you started which is important because I think the earlier something gets programmed into your brain or even your complete being, the better it is.
But there are other things that are hard to measure. Desire...some athletes are happy making a living and would rather play for a team in a small market, collect their paycheck and be almost anonymous. Other crave the spotlight and want to be the star. It's also grace under pressure. I'm sure there are potentially great athletes that have the talent, but lose it when they see more than 20,000 watching what they do.
It's hard to compare sports with entertainment because sports is performance oriented. If you're not good at your job, someone else will take it. In music, you don't have to be "good". You can be "cute", you can be "quirky", you can be a lot of things that help you be successful that have nothing to do with having an actual talent for singing, or playing guitar, etc....