They don't fail to control radio, they just don't control as much of the radio.
Radio is still a weird medium is probably why, and it does vary from market to market. Back in the Bay Area, radio was hugely left-wing, while here in SA, even TV is a bit more conservative (actually, I find it a lot more balanced - there was no balance in CA).
The media is a business. They need "customers" who love them and they need shock and awe to keep their attention. Any way to divide and issue into 2 greatly opposing sides is a chance to incite emotion and get attention.
The media isn't what the media was born to be. Like everything, it's about the $$$.