Because all electronics broadcast signals. As I said in my last post, it is posible that many non-broadcasting devices are actually much more dangerous than mobiles because they broadcast noise at the frequencies many aircraft systems operate at at much higher power than things like phones. This will mainly be a problem for high power things like computers and I don't imagine a radio could cause any problems. However, the approach is better safe than sorry. Many electronic devices can cause problems, and allowing any on board would require testing them to ensure they are safe. Since this is just not feasible to do for all possible things that people might have with them, all electronics are banned. It may not be fair, but in my opinion, and apparently that of airlines, it is better to have people go a couple of hours without their phone and computer than to risk having planes dropping out of the sky.
When it comes down to it, is it really that important that you listen to music or play computer games while flying? What's so bad about just reading a book?
Incidentally, are radios actually banned anyway? Most airlines allow passive electronic devices like mp3 players while in flight, just not during takeoff and landing. I would assume radios are the same.