Why do the living creatures and the elders now sing?
Revelation 5:9 (ESV)
9And 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,
As in the Old Testament temple, praise befits the all-glorious God, enthroned in Zion on the praises of Israel (Psalm 22:3). With the Lamb now being judged worthy to possess the scroll, heavenly liturgies of praise rightly may be directed to him.
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,