A 2* high school player has more time to develop and is more likely to make it to campus and stay in the program, whereas a JUCO player has limited eligibility remaining and may have grade issues. Keep in mind players still have to make the adjustment from playing at the JUCO level to major college FB. They've typically not going to come in and star right away... they still have to learn your team's systems, etc.
If you recruit a JUCO and they either don't make it to campus or bomb out, or just end up being practice depth, it really puts you behind the eight ball in terms of recruiting and roster depth. That's what really hurt Cam Cameron at IU in his last couple seasons... he went after JUCOs hard and a lot of them didn't work out.
Basically JUCOs are good for filling holes, not building a whole team around.