Numbers 7:9
But unto the sons of Kohath he gave none: because the service of the sanctuary belonging unto them was that they should bear upon their shoulders. But unto the sons of Kohath he gave none: because the service of the sanctuary belonging unto them was that they should bear upon their shoulders.
The sons of Kohath were given none of the carts or oxen offered for the service of the tabernacle, because the sacred charge belonging to them was to bear the holy things upon their own shoulders. The ark in particular was to be first covered by the priests and then lifted and borne by its staves, and it was never to be placed upon a cart for transport. When Israel brought the ark from Kirjath-jearim upon a new cart, they set aside these plain directions and thereby dishonored God. The sudden judgment that fell upon Uzzah for that disregard deeply impressed upon the people that God's commands regarding holy things require exact and reverent obedience.