It is all about the content folks. Much of WCPT's content is brutally dull and monotonic. Not to mention that liberal listeners in Chicago tend to prefer the likes of WBEZ (which does do well); and they have a multitude of other choices on radio, TV and on the internet. This is one reason why liberal talk doesn't do that well in any big market and why it has never held a candle to conservative talk. Different audiences with different tastes, AND different competition.
As far as WCPY goes, what b is saying is true* (with an asterisk). No, it doesn't cover the City of Chicago (as in the place where Daley is mayor). However, from my work here, I can pick up a weak signal from them. You can't just toss that signal out - that's absurd! And Rockford has nothing to do with it either. The 92.5 signal is excellent in DuPage, Kane, Will and other western counties. These are some fast-growing and densely populated parts of the market. 99.9 covers the city; 92.7 covers the north suburbs and 92.5 the west. All do fine. Of the 3, 92.5 covers the most territory.