Actually, the “why” is quite easy. Radio requires you to think. Thinking is the enemy of the Progressive. It requires a blind faith adherence based in emotion. Raw emotion without Reason can only get you so far. Contrast this with television, where ideas and thought invariably take second place to the imagery. Imagery is very easy to manipulate to get the needed emotional response. Hence the preference of Progressives in the visual media.
The know this and use it.
If you have any doubt? Consider this; with the push for reestablishing the Fairness Doctrine for radio have you ever noticed there is never an effort to apply that same standard to television?