Maybe it has been mentioned here, but I've heard a few reasons they don't want to go to radios. (For the record, I think the NCAA should allow the radios in the helmets.)
First, is the cost. Not all teams make money, so the teams that don't make money - here's another thing to bring their bottom line even farther down.
Second, for some reason (I didn't hear what that reason is), that move would first be done on a conference-by-conference level. So if the Big Ten gets radios, and the Big Twelve doesn't, they're going to cry that the Big Ten team has an advantage. So.. fine - they can't use radios when playing out-of-conference opponents. It's now a disadvantage for the Big Ten team that has been used to using radios to suddenly not be allowed to use the radios. So the conferences don't want to put their teams at a disadvantage against other conferences. Again, I think the NCAA should just allow it across the board, but whoever came up with this seems to think that won't happen.
Third reason I heard was about liability. I don't know why this would be different from the NFL, though. But the helmets are made by some company, and that company doesn't want to get into the liability of including radios. The radio company doesn't want to get into the liability of college kids' safety. Seems pretty lame to me - why not use whatever company is making the helmets for the NFL?
Finally, I think some teams don't want to go to radios because they've gotten good at legally stealing the other teams' signals, and breaking their code. I'm saying "legally" because I don't want to know about who else is illegally doing this, though, I think we'll hear about more teams in the next 3-5 years. But these teams get a good advantage by breaking the other teams' codes, and they don't want to give that up.