They mount them sideways to help dampening through corners, and to give the shocks a longer stroke over a shorter distance. If the shocks were straight up and down, the tubes would be shorter, making less room for internal baffles for the oil, or gas, or whatever is in the shocks to help with the dampening. When the shock compresses at an angle, it moves farther to cover the same amount of distance, increasing the dampening effect. It also controls side to side movement of the axle, if the axle sways side loading the leaf springs, the shocks compress and expand, where a vertically mounted shock would just load up the bushings.