why are no new, non-personal songs added?

It is unclear. Perhaps something has been lost of their culture. They no longer notice or value the same things and moments that they used to,

"His people had once been great makers of songs so that everything they saw or thought or did or heard became a song. That was very long ago. The songsremained; Kino knew them, but no new songs wereadded. That does not mean that therewere no personal songs. In Kino's head there was a songnow, clear and soft, and if hehad been able to speak of it, he would have called it the Song of theFamily.His blanket was over his nose to protect him from the dank air. His eyes flicked to arustlebeside him. It was Juana arising, almost soundlessly."