Radially laced spokes have an inherent geometrical advantage that will result in better lateral stiffness compared to crossed spokes. However they are very poor and possibly dangerous for transmitting torsional loads, which is why the disc side spokes need to be crossed, to transmit braking forces to the rim and tyre. Because the disc requires room to be mounted on the hub, the flanges and spokes need to be moved further towards the centre of the hub. This further worsens the geometry they are working with, in regards to resisting lateral forces on the rim, making that side of the wheel less laterally stiff.
So the disc side now has significantly worse lateral stiffness due to the double whammy of worse bracing angle and spoke arrangement. In order to counteract that and attempt to even out the lateral stiffness on both sides, you can reduce the number of spokes on the non-disc side by half (it can't really be more or less than that unless you want to create the mother of all headaches to design the rest of the wheel). The Roval CLX II nailed this almost perfectly with 18 spokes laced 2:1 and very even lateral stiffness
