In the '70s, radio disc jockeys had a great deal of influence over the music they played, and could help out artists they liked by playing their songs. Very often DJs would have to work for many different stations throughout their careers as stations would change format or "direction" and let them go. These guys would often trade a stable family life for the excitement of being on the air, which could be more difficult as they got older.
Chapin was constantly promoting himself, and made sure to visit radio stations as he traveled across the United States. The song is a composite of the disc jockeys he got to know, including Jim Connors, who was an air personality at a radio station in Erie, Pennsylvania, and in 1973, became the morning man on WYSL in Buffalo - he had something of a freeform style which endeared him to many listeners.
Chapin was very astute from a marketing perspective (he went to Cornell, you know), and figured out that if you were trying to get disc jockeys to play your song, it would be a good idea to write a song about a disc jockey.