Why does Rush do well in Buffalo? He's sandwiched between a lot of local content and backed by the only commercial news department on the old-line full-service station in town. Hannity? He's on after 7PM, which TheBigA has identifid as throwaway time in radio anyway. WBEN goes from 3rd to 7th in his target audience when Hannity's on, so even promotion and placement don't help him. Put those two on 'KB, and they might break a 1 share because they've been on in town on one of the market leaders for so long.
Would any of the "progressive" hosts fare well on WBEN as an alternative viewpoint? Nobody knows, and likely never will. Ron Dobson was relatively centrist, and there were complaints that he wasn't conservative enough. Those of you who point out that people tune into hosts who reinforce their point of view are correct. I suspect that the number who tune in to talk stations to "hear the other side" is miniscule. And, yes, Liberals are every bit as jingoistic as Conservatives.
What may be true is that younger audiences may be listening more on-line and on-demand, which is why they didn't show up in 'KB's audience. Or they simply don't listen to blather from either viewpoint - which I find more likely.
Lastly, NPR is hardly "Liberal". Virtually every study conducted has determined that NPR is more centrist than any other media outlet these days. Will both sides be represented? Generally, yes. Will they presented with equal fervor by equally talented presenters? Sometimes. At least they try to get multiple points of view on the air.