How is it a monopoly? Nobody is stopping anyone else from launching a satellite and starting their own service. This is different than the situation we have with cable and telephone, where one provider owns the entire infrastructure and it would be cost-prohibitive to start your own. Think it couldn’t happen? Sirius did it when XM was the only game in town, and now their marketshare is close to 50%.
Plus as long as commercial radio exists, there is still competition. You either choose to pay for satellite service, or not pay, and listen to about 50% less actual content on commercial radio. A monopoly means you have no choice. Even cable is not a monopoly, as nothing is stopping anyone from going to a DBS setup.