When I know, I'll tell you..! 😅
At a guess:
Probably a combination of dedication, commitment, tenacity, guile, experience and a good support network.
Other things which probably wouldn't hurt are a firm grasp of numbers/maths/probability and a generous sprinkling of luck?
Natural talent/ability will perhaps produce remarkable players like Stu Ungar but I don't believe that is
essential to be the best, it can be learned. 🙂