There are no guarantees for anyone–even for those who have a strong application portfolio. There are simply too many well qualified applicants and too few seats.
This is why every pre-med needs a viable Plan B career in mind.
While not a primary determinant, I would say there is some element of luck involved.
Students need to be savvy about where they apply and how they present themselves. But they have no control over who is assigned to be their application reader or interviewer (should they get to that stage). They also have no control over the quality of the other applicants they will be competing against for the limited number of seats offered by any given med school.