It's probably a combination of things. For one, I believe the rise of right-wing talk on AM radio stigmatized the band to such a degree that liberal and centrist-minded listeners abandoned it, never to return even if progressive stations were to pop up there now.
Along the same lines, conservative-minded people tend to embrace things the way they have always been while progressives tend to evolve more quickly. This is a generalization of course, but it's actually part of what defines those groups. With that in mind, AM radio is a technology of the past that seems to appeal to old conservative men who grew up with it while more progressively-minded people moved on to the internet and more modern technologies long ago. Of course the internet offers opportunities to intensively read too, along with books and in-depth newspapers like The Washington Post and New York Times. I believe that type of media competes more with radio on the left than on the right.
Next, the programming style of public radio caters well to a liberal/progressive audience that appreciates a long form, intellectual conversation with experts on both sides given ample time to make their case. The one-sided, red-meat slinging style that works so well at conservative talk has typically failed when attempted by a progressive outlet, at least on the radio. Plus most NPR stations are on FM avoiding the disadvantages and stigma of the AM band.
Finally, a conservative audience is more monolithic in its thinking than the liberal/progressive side which is made up more of various factions, so a one-size-fits-all style of programming doesn't work on the left as well as it does on the right. I think it's fair to say that a brand new liberal/progressive network has just popped up under our radar this month but you're probably not even thinking of it in those terms until I mention iHeart's new Black Information Network. It laser-targets one of those Democratic factions and is likely to be more successful than the umbrella approach that was taken years ago by Air America as a network that tried to appeal to all liberals.