“But he did not give to the sons of Kehat, because the sacred service is upon them; they shall bear on the shoulder” (Numbers 7:9). “But he did not give to the sons of Kehat…” – Rabbi Natan says: From here, that it escaped David’s mind, as the Levites bore the Ark in nothing other than a new cart, as it is stated: “They mounted the Ark of God onto a new cart.… David was distressed because the Lord had lashed out in a breach against Uza” (II Samuel 6:3, 8).
Aḥitofel said to David: ‘Should you not have learned from Moses your master that the Levites bore the Ark only on the shoulder, as it is stated: “But he did not give to the sons of Kehat…”’ David then sent them, and they took it up on the shoulder, as it is stated: “David called for Tzadok and Evyatar the priests, and for the Levites: For Uriel, Asaya, Yoel…” (I Chronicles 15:11), until: “In accordance with their practice given to them at the hand of Aaron their father” (I Chronicles 24:19), “[The sons of the Levites bore the Ark of God upon their shoulders] as Moses had commanded in accordance with the word of the Lord” (I Chronicles 15:15).
Where did he command? “But he did not give to the sons of Kehat…” The Levites did not initiate anything, but rather everything was from the mouth of Moses, and Moses from the mouth of the Holy One blessed be He.