John Gill Commentary Numbers 7:9

"But unto the sons of Kohath he gave none, because the service of the sanctuary belonged unto them; they bare it upon their shoulders." — Numbers 7:9 (ASV)

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) .