To answer your question, not everyone is online and of those that are, how do they know where to find the online information? Radio and TV are still the best ways to drive online traffic. More Americans are still reached by radio and TV each week as in percentage of the total population. I'm not trying to discount online influence, it is substantial but not yet what broadcast media enjoys.
I did a study of online listening to college radio and found that numbers were dismal. When a college of over 11,000 can only manage 500 listening sessions a week on a well programmed online station run by the college students themselves and well promoted on campus, you have to wonder why they can't achieve greater than 750 listening hours in a week. For most, a show getting about 10 listeners at a time is doing quite well.
The best excuses I can think of are: with over the air going to college usually means moving away from home so you have to develop new listening habits. Online you don't have to. On line is not geographic based. And there are just so many options plus no central location to find all the offerings. With on air, it's it's not found on the dial, that's it or in other words, the limited options mean more people listen to any given on air station than the online counterpart. From studies I have seen, many listeners online prefer to listen to streams of over the air stations because they feel connected to the community where they live and work. They say they miss that on an online only station. That begs the question as to why there are not more online stations targeting certain geographic locations? Is it the high cost of building awareness and staying top of mind?
If online Pandora and Spotify listeners will have to pay to listen and/or deal with a commercial load that might actually have to mimic over the air radio, will people pay or deal with commercials in numbers they currently enjoy. I personally think not. I think the threshold of pay to listening is probably what Sirius has managed to achieve. I could be wrong. Who would have thought people would pay for cable TV or for that matter buy water back when those ideas were hatched?