It's simple math, really.
When players are determining whether they should turn pro, one NBA executive who deals with player personnel wants them to be truthful in answering a couple of questions.
How many times were you the best player on the floor?
In how many games did you dominate your peers?
If those numbers, over the course of an average 30-game season, aren't reaching into the 20s, then it doesn't add up to declaring for the draft.