Numbers 7:9

But unto the sons of Kohath he gave none
Being all disposed of to the sons of Gershon and Merari: the reason of which follows,

because the service of the sanctuary belonging unto them [was that]
they should bear upon their shoulders;
as the ark with the mercy seat, and cherubim, the shewbread table, the candlestick, and the two altars; though in later times we find the ark was sometimes not only carried by the priests, but upon a cart, ( Joshua 3:17 ) ( 4:10 ) ( 2 Samuel 6:3 ) .