Because they can't stop them under the Constitution. Not yet, anyway.
If Hillary gets in, I would expect right-wing talk radio to be taken off the air. They've already tried several times.
Hillary will pack the SCOTUS with more far left-wing activists, and that will be the end of constitutional limits on the government.
Of course, the destruction of the 2A will be the opening shot of CWII, so she'll have her hands full with other matters.
Sure I-Heart could stop them. If I-Heart stopped syndicating and carrying them, many would atrophy or struggle.