Why is a new song sung?

Revelation 5:9 (ESV)

9 And they sang a new song, saying, “Worthy are you to take the scroll and to open its seals, for you were slain, and by your blood you ransomed people for God from every tribe and language and people and nation,

The Lamb had just taken the scroll from God’s hand, which is a new development in the history of redemption. The elders recognize the novelty of what had just happened by generating a new song. This new song is invariably built on old songs (from the Old Testament) but did justice to the new developments they had just witnessed.