The focus of these concluding Psalms (146-150) are on the "who's, what, when and why" of praising God. Psalm 148 focuses on the "who's". In the first two verses, the text focuses on those in the heavens praising God.
This is not a reference to people who have died. This is about other entities that God has created and these verses ask them to praise Him. But John, you said that angels couldn't sing. The text does not ask them to sing, but just to praise Him.
Why should we care whether or not angelic beings praise God? The idea is that if we all are to be grateful for this world that He has created, we ask that whatever entities do exist in God's world (i.e., heaven) also appreciate the world He has created.
These verses are not a demand, but a request. God wants us to glorify Him and the psalmist is asking us to ask angelic creatures to join that praise. So why should those angels join us? It is for them to appreciate the world God has created in the same way He asks us to appreciate the world He has made for us.
One may ask if there is a difference between an angel and a heavenly host? They are probably different ranks of angelic entities. The text is saying in effect, may all the angels and other things God has created in heaven appreciate the work He has done and praise Him for His work.
Remember that Satan himself was once a top angel. The Bible teaches that one third of the angels choose to rebel with Satan. Why did they rebel? That is debated, but I believe they wanted to glorify the creation (including themselves) over the creator (see Ezekiel 28:13-15 and Revelation 1:20 and 12:4).