I think the number one factor is the prestige of the school compared to the ranking of the plsayer - if a kid is delighted to be recruited by a school, he is more likely to enroll - if he is a reach, less likely
second - randomness
third - effort spent - I'm convinced it has an effect, less that #1 above
fourth - MAY be that some kids are more likely to juco - is it in the same sort of traits as predict redshirt??? dont know