My understanding is this:
1. Junior College players must either graduate from JC or they have to have been an NCAA "qualifier" out of high school (that's an NCAA rule, not ISU).
2. JC players must have a 2.75 JC GPA to get in to ISU, but the 2.75 is only in their "core" classes (throw out the PE credits and fluff that you get a lot of at JC). The NCAA, I believe, requires a 2.5 cumulative JC GPA including all courses, that's where it's harder for ISU. It's a lot easier to have a 2.5 with 12 PE credits of A's.
3. I think that's it. Coach Evans has the same recruiting guidelines as Coach Kramer and he recruited all JC's in his first class. So my guess is it's a decision the football coaches have made.