I have no data to back this up, but based on my own experience* I think there are differences in the nature of communication between cell phones and radios (CB/Amateur/business/etc.).
Cell phones being full-duplex, people are more likely to talk over one another, as they do in face-to-face conversations, which means you have to be listening while talking. With half-simplex radios, on the other hand, you're either receiving only or transmitting only, which seems like it would make it easier to "compartmentalize" the activities in the brain. Again, I'm just hypothesizing. Also, on the radio, long pauses in a conversation are not on acceptable but frequently encouraged (to listen for any other stations to want to join the conversation or use the frequency). This, coupled with the necessity of coordination would seem to have the effect of generally slowing down the pace of the conversation, and freeing up more cycles in everyone's brain.
*Granted, I'm definitely biased on this topic, being a ham radio operator who regularly talks on a mobile radio with several other hams while driving to work in the morning. I do try to take sensible precautions, though; I don't talk and drive if the weather is bad, if visibility is low, if I'm trying to navigate in an unfamiliar part of town, or if I'm merging or performing some other delicate maneuver.