unto the: Because they had the charge of the ark, table, candlestick, altars, etc. (Num 4:4-15), which were to be carried upon their shoulders. For those sacred things must not be drawn by beasts.
because: Num 3:31, Num 4:4-16; 2Sa 6:6, 2Sa 6:13; 1Ch 15:3, 1Ch 15:13, 1Ch 23:26