I was also surprised by how easy (and free) it is to get a podcast on iTunes, especially compared to getting an app on the App Store.
So this was a case where Apple sort of came along for the ride and didn't see podcasting as being a big deal--which they were sort of right about; arguably even today it's somewhat mainstream but I'd bet the majority of people in the US have never listened to a podcast and certainly don't listen regularly.
A few weeks later my dad calls to tell me he doesn't know how any of this works, but now he gets amazing content while driving, instead of just listening to the mostly ads that came through his antenna.
So.. just lack of awareness? Just fiddly enough to keep people away?
I have a sneaking suspicion that if tomorrow Apple or Google just subscribed everyone to 99% Invisible and waited, FM radio would be dead in a year.
Many people don't want to have to deliberately choose content a lot of the time. I've heard people argue that they don't want to always have to use Netflix because they can't just flip on a channel and/or channel surf.