In the early '80s, when Washington first started cutting funds for public radio, cash-strapped stations faced two possible paths. They could aggressively seek out business underwriting, do a lot of marketing research, and, in general, emulate commercial stations. Or they could become less costly and less bureaucratic, and more dependent on volunteer labor and listener sponsorship. Most chose the first path -- in part because the government decided to subsidize it, reasoning that outlets that could attract large audiences and substantial private funds were the ones making enough of a difference to deserve federal subsidies.
Why, you might ask, should a successful mainstream station serving an affluent demographic get any subsidies at all? Good question.
Jerry Starr has written a nice piece on the split between the two sorts of stations and the odd politics of the public broadcasting game. It's worth a read, and not just because he had the good taste to quote a paper I once wrote about the topic.