I think it boils down to a few things:
1. JCs aren't a sure thing to get into a school like Iowa that has some academic standards. Most are at JCs because they couldn't qualify to begin with or because they did something to scare away everyone including the schools that routinely take character risks. There are exceptions of course. Iowa has had some success with JCs during the KF era. One of the top NFL QBs was a JC kid but for the most part, JC kids are at JCs for a reason and they usually aren't good.
2. Iowa is a "developmental" program. JCs have 2/3 years rather than the 4/5 for high school kids.
3. With JC kids, they often need summer classes to get themselves qualified. So you can put yourself in a tough situation by giving a kid a scholarship in Feb with with hope that he does his classwork for the spring semester and then the summer term(s) which keeps him from being at Iowa's summer conditioning program that high school kids are routinely at or even running into fall camp that begins at the beginning of August.