Talent and readiness. The readiness factor is hard to determine.
Ability to make adjustments, ability to handle a wide variety of pitches that MLB throw, and other mental and emotional factors that not many of us know about.
Here's something from soxprospects.com 6/19/17:
With Red Sox third basemen still struggling, the calls for Rafael Devers to come up to the big leagues will only continue to grow. After seeing Devers for three games recently, however, it is clear he still could use seasoning in the minor leagues. Devers can get pull-happy and overly aggressive hunting fastballs. As a result, he can be susceptible to off-speed pitches and extend his zone, chasing them. Even when he briefly falls into such streaks, Devers has the mentality and ability to turn things around quickly. He has shown this ability recently—after an 8-for-48 stretch over his final 12 games of May (.167/.226/.292, 1 HR), he is hitting .364/.397/.764 with 5 home runs.