Virtually everyone who works behind a radio microphone has been fired or laid off at least once, maybe twice, quite possibly more than that. Sometimes an on-air host is sacked because he or she isn't good. But often they're let go simply because not enough people are listening. That's because in radio, as in television, ratings are money. When the dollars disappear, people are told to scram.
Radio is, of course, free to listeners.
But someone has to pay for the disc jockeys, the radio station bosses, the offices, the antennas, and even those annoying billboard ads. At for-profit stations, all that money comes from commercials. And the advertisers want a lot of bang for their buck. You might think a station with few listeners would charge less for commercials than one that's more popular. But in these days of highly specialized advertising, it's not that simple.