They carry conservative hosts because if they don't other stations will.
The Clear Channel has 1200 stations and distributed Rush. They have a lot of conservaitve hosts. Some are local like the ones on WLW in Cincinnati.
But if tomorrow some liberal host started drawing audience and could out draw Rush, he would be on and Rush would be in a new time slot.
It is mostly a commerial venture. Conservative hosts garner audience. They sell audience. If a host does not garner ratings he or she is history.
It just appears that in the market place of ideas, conservatives win audience with a score of about 90 to 10.