I think part of the resentment stems from the fact that many (not all but many) are borderline violent or seditionist in their rhetoric, if not on the air on their social media presence, in terms of what they would like to see happen to liberals or even "RINOS." When the side with more guns literally talks as if they'd like to extinguish you, and they have the loudest voice at least in terms of talk radio, it feels not only imbalanced but literally threatening. Usually it isn't the left showing up at school board meetings and town halls with "rope, tree, journalist - some assembly required" shirts. Yes, the left has extremists too. But you hear a lot more of that violence and revolution winked at on the AM airwaves than the likes of NPR, or a random community station that carries Democracy Now and Thom Hartmann.
Also, while NPR rates well in metro areas, there's a heck of a lot more AM stations between the coasts carrying influence with one side of the narrative to the people that have the guns and form the militias. So it isn't just "contrary voices" - it's that some people, particularly marginalized people without much leverage, literally feel threatened. And it isn't reasoned advocacy for lower taxes or debate about federalism versus states rights that causes that. It's actual inflammatory rhetoric. To a degree that far outpaces any voice for leftist radicals. I have heard some great conservative hosts who were funny, compelling, and not hateful. But they weren't the ones stockpiling ammo and conflating every lifestyle they didn't understand with a globalist or Communist plot, or talking about trials for NBC News anchors/reporters.
Conservative talk radio doesn't make every moderate or liberal angry. It makes a lot of them sad, or fearful for established norms and existential safety.