Why is that?
I can offer at least one opinion, and that is that the Progressive Leftist ones are grating, arrogant, annoying, and shrill, among other negatives. For example, Al Franken set up Air America, a Progressive Leftist media group that was to counter Right wing talk radio. I tried to listen, but found I couldn't take the inane, often vulgar, insulting, grating style many of his show hosts had.
Stephanie Miller came off as a buzzed (she claimed to be drinking while doing the show more than once), college freshman who thinks scatological humor and insulting the opposition was hilarious. It was puerile and juvenile.
Randi Rhodes was just vicious. She got tossed off the air one time for making threats against a sitting President (the Secret Service paid her a visit), Bush II. She was just so grating and vulgar you couldn't listen for more than 15 to 30 minutes.
Ed Schultz was another blowhard on this station. He passed away a while ago. Two that I remember from his show were once he advocated voter fraud, then when called out on it doubled down while trying to make it into a joke. The other was he once brazenly challenged Dennis Prager, a Conservative talker, to a debate. Prager responded with Name the time and place! Prager even went so far as to offer to pay all expenses of Schultz to participate. Schultz quickly started hemming and hawing then backed down. I thought after that he was nothing but a spineless bully.
Franken's own show was nothing short of stupid. It was like bad open mic night at a comedy club where every person taking a turn could talk about nothing but politics. It made you want to leave.
The only one that seemed reasonable was Thom Hartmann. He really tried and used much the same format as Conservative talkers did. I didn't always agree with his views, but at least they were reasonable and debated on his show.
Anyway, that's why the Left fails so badly at commentary. They bunk no real debate, and when presented with any opinion other than their own get rude, vulgar, and vicious toward the person offering it. Who wants to listen to that?