Whereas Psalm 148:5 looks at the power of God as creator, Psalm 148:6 emphasizes that he is forever the One who maintains all that was created. When God had completed His work of creation, he continued the work of maintaining and governing everything.
The reason for the praise lies in his work as Creator (Psalm 148:5) and in his work as Maintainer of all creation (Psalm 148:6). In this verse it is said that the Lord has decreed a place for all heavenly creating beings and things. In Jude 1:6 (compare with 2 Peter 2:4) it is said of the angels who fell into sin that they left their proper dwelling and did not stay within their own position of authority appointed by the Creator. What is written in Psalm 148:6 is not just an observation; for the angels it is also a prescript. The fact that some angels did not follow this prescribed position has had disastrous consequences, for themselves, but also for man, for the greatest of the angels who fell into sin has tempted man to sin.
6 And he established them forever and ever; he gave a decree, and it shall not pass away.