John Gill Commentary 1 Chronicles 15:2

"Then David said, None ought to carry the ark of God but the Levites: for them hath Jehovah chosen to carry the ark of God, and to minister unto him for ever." — 1 Chronicles 15:2 (ASV)

Then David said
Either within himself, or to his ministers and courtiers about him:

none ought to carry the ark of God but the Levites ;
he saw his former mistake in employing those that were not Levites in bringing up the ark from Kirjathjearim, and bringing it in a cart, and not on the shoulders of the Levites, and so miscarried in his attempt, (2 Samuel 6:1) and, perhaps, had since more diligently consulted the law of God about this matter:

for them has the Lord chosen to carry the ark of God, and to minister
unto him for ever ;
as long as the Levitical dispensation lasted, as appears from (Numbers 1:50) (4:15) (7:9) .