I don't work in radio, I'm not a radio historian, so there's a good chance I'm violently wrong- but my pet theory is that advertising killed radio in the US. You have to fit into a small number of surprisingly rigid categories in order to participate in the big money advertising campaigns, and if you don't have those, you're liable to go dark.

There are exceptions, like the Pacifica network and oddities like KPIG, but they're few and far between.

If you're looking for some corroboration, you could check out documentaries about the early days of hip hop.