The problem is that online radio is competing with Facebook, Instagram, Snapchat, Twitter, YouTube, Pandora, Spotify, etc. when it comes to millenials and younger demographics.
Another problem is the economy. When people have less disposable income, they don't pledge as much money. Commercial radio is feeling the pinch -- I haven't seen the numbers, but I'm sure NPR feels it also.
Even though their listeners are in the higher income demos, those higher income demos are probably not doing as well as they did before 2008.