Perhaps people would always have preferred those kinds of channels, but in the past they just weren't available.

Talk radio's been around for a while, and TV pundits only a little less so, so I'd hesitate to blame either one. The political blog scene might be more directly involved; it's highly polarized, has excellent visibility among politically aware individuals, tends to be kind of incestuous, and coincides roughly with the 10-year timeframe we're discussing.