The big companies are focusing on developing the podcast business because they see it as having far more room for growth than many anticipated. New technology has made listening more accessible. New business models are opening up opportunities for a middle class of podcasters. And podcasting’s potential for expansion seems relatively endless given how little it’s encroached on its analog alternatives so far.
The good and dark side of podcasting (attention economy) is that you can consume the product while doing other things. With text and video this is not possible, for example if you are walking, driving, cooking, etc. So it may be an untapped market.
Anyway, there is a lot of free room left because for every 100 minutes on Spotify, people spend 40 on radio and only 6 on podcasts (see this research).
The difference now is the technology, very easy for the user, just a button on the smartphone. Before it was hell. That's why it is estimated that AM/FM radio is going to be eaten up. We will see.
For further details and more in-deepth analysis read this post.