Progressive talk has generally failed on AM for three reasons. It's done a little better on FM, but securing carriage on an FM station, especially in a large or major market (which usually would mean buying an FM at a cost which may exceed $10 million) is generally beyond the reach of anyone trying to get such a venture going.
That said, AM liberal talk was hobbled in four ways:
Liberals/Progressives find great importance in demonstrating that their lifestyle (and thus their agenda) is the most modern, up-to-date, and formed from reason. They consider "Liberal" to be pejorative and prefer the term "Progressive", they make sure their phone is the latest and most svelte iPhone, wear the latest fashion, and wouldn't be caught dead with a CRT television in their house. Listening to an AM station would fly in the face of that post-modern image.
More liberals/progressives (as compared to conservatives) tend to work during the day in an office environment, likely in a metal structure, in the presence of computers and other appliances that produce RFI. They could not listen to an AM station at work if they wanted to. More conservatives (especially "Tea Party"-leaning) work from small business or from home (especially those who have their own small businesses), where AM reception is better.
Liberals/progressives are more likely to receive opinion from non-broadcast sources, being more likely to read traditional print newspapers and magazines, and selectively surf internet sites and audio podcasts for opinion.
Liberal/progressive talk radio came late to the parade. Conservative talkers had already established their foothold on the 50kW clears and mid-market Class III-A regionals who had abandoned their full-service adult formats. Liberal/progressive talk usually wound up on AMs with real coverage issues. My sister was an interesting case-in-point. An unabashed progressive, she was talking to me about what she thought was a "conspiracy" to keep people from listening to progressive talk. She pointed out (correctly) that WDTW 1310 (now defunct) was riddled with interference at her St. Clair Shores home after sunset, while conservatives were still loud and clear on WJR. I had a hard time getting her to believe my explanation of how not all AMs were equal, skywave interference at night, clear channels as opposed to "junior" regional stations, and directional arrays, but she did eventually take my word.
As for NPR. I would agree they are slightly left-of-center, more in some ways than in others, representing the well-to-do liberals/progressives more than the old-fashioned "rank and file" liberals, but I still listen to NPR (usually via WUOM). I consider NPR a good service, at least mentioning the issues, in and outside this country, that other media outlets ignore. Like any journalism, I take everything I hear with a grain of salt, and, if I'm really interested in a story, I'll seek out dissenting opinions.