A bigger question is why the left has, with very few exceptions and NPR, failed at talk radio? Probably because you're only going to get a limited audience/TSL when all you do every day is bash the other side. What happened to each side presenting their ideas and trying to win the public to their view?
NPR is not "liberal talk radio."
Most liberals I know find it smart, good at news, but not really "entertainment" in the way people often use talk radio.
As to a limited audience with only bashing one side all day...might I introduce you to pretty much all of talk radio for the past few decades. And it's practically all conservative, so you answered your own question. That worked, as liberal hosts either retired or got canned, they didn't hire or develop new liberal talent.
(yes, they tried with Air America, there were a myriad of issues there - lack of actual talent development, the demand that affiliates carry the
entire schedule, and the financial mismanagement.)
Emmis tried to plan for the post-Limbaugh era by developing local hosts that were more centrist to left, they had a very successful one in Jeff Ward (who actually wasn't a liberal, just a no nonsense libertarian who didn't buy the hot air) and Dale Dudley who started to gain traction in his time slot, and was even asked to consider expanding his show, before he kind of imploded for personal reasons and didn't want to keep doing it) and then, Sinclair and Waterloo took over and basically made all the non-Trump fans in the building shut up about politics.
So now they're riding that conservative talk horse till it faints. I know some of it was business, some of it was money, but some of it was bias. Why would you want to develop local hosts that would question the conservatives when Fox News has proven that the audience wants a steady diet of their own views? They're not coming to the church to be convicted, they're coming for the pep rally. Even Fox took fire for daring to have Shepherd Smith and Chris Wallace not toe the line. There's too many headaches and not enough guaranteed revenue in an uncertain time so this is what we get.
Personally, I loved the format in the 90s when you'd get conservatives, libertarians and liberals, all bouncing off of each other and taking calls, doing lively radio. WLS, WSB, KFI, they all did this. But times change and consolidation happened, and frankly, businesses are ultimately conservative. If it's a risk, it costs money and you're catching heat from the audience for daring to have someone on air even questioning Trump (Ask Joe Walsh and Michael Medved how welcome they are at Salem) then why would you do it just to give someone airtime who's going to want to raise your taxes and have businesses regulated? There's nothing in it for them.
I respect what Charlamagne does a lot, but he's a general show on a music station, not a daily current events political show. And he's in a format that has a tradition of taking on community issues in a way that your average classic rocker or country outlet doesn't. So it works there.
But for talk, you're basically left with:
1. Conservative talk (most AM (and FM) talk outlets)
2. NPR (center to vaguely culturally left and ultra proper, not really entertainment)
3. Podcasts and streaming (if you're liberal or non-Trump conservative)
The existence of a daily, entertainment first (think Jon Stewart) live talk radio show hosted by a liberal who isn't too "proper" on commercial radio, to my knowledge does not exist on commercial radio in the United States at any significant scale or rating. Sure, Stephanie Miller is still on a few stations but the days of Jay Marvin, Mike Malloy, and others are done. I'm a fan of KMBZ in Kansas City, and used to enjoy their afternoon team bantering on current events. Dana Wright was the liberal, Scott Parks was a non-Trump old school Republican. They got so sick of the hate texts and calls they leaned into local and are doing fine, but their insights and friendly but spirited debate on current politics was enjoyable. If I want "liberal" talk that isn't prim and proper, I have to tune into LBC in London and get it from James O'Brien who isn't afraid to take on callers with some bite.