Rear wheel drive side spokes are generally built in a crossing pattern because of something called wrap up, which is the effect created when you torque the rear wheel by stepping on the pedals and of course by applying the brakes(on the front also). When spokes are crossed over each other, they leave the hub in a tangenital direction, or in the direction that the torque is applied, delivering the force from the hub to the rim with a minimum of hub/spoke wrap up. Wrap up is obviously wasted force, so the elimination of it is a desirable thing. The more crosses, the more tangenital the spokes are to the hubs, but the increasing efficiency is offset by the increase in the length of the spokes which produces more flex and more weight. So...drive side spokes in a traditional wheel that is built for lightweight, hill climbing or time trialing applications are usually built drive side 2x, radial on the flip side.
An example of a very light climbing wheel from back in the day might be an Omas or hi-e hub with ti axles and ti skewers with aluminum q/r nuts, Super Champion Medaille d' Or rims(260gr ea.!), and 15 straight guage spokes, 24-28 radial frt, 28-32 2x/radial rear. In more recent applications, rims, spokes, and hub flanges are being built heavier or thicker to withstand the use of fewer spokes, ergo they are also able to build drive side radial, since heavier or thicker = less flex and less wrap up. In some hubs now, all the spokes leave the hubs in a tangenital direction without a spoke head in a spoke hole, which also eliminates wrap up. Most wheels with nipples in the hub are built this way, and all hubs which take spokes that have no bend at the spoke head are like this, so wrap up is completely eliminated, eliminating the need for crossing. Larger drive side flanges do even out tension and tangenital spokes do eliminate wrap up. These are well established principles of wheel building. All the various wheels on the market are different applications of these fundamental principles.
This is why you don't see a traditional style rim like the Mavic Open Pro used in the low spoke count, radially spoked wheels. They are too light. Also, Campagnolo Record hubs are still not warrantied for radial use, since they are a traditional hub with traditional flange thickness. A radially spoked wheel built to tight on these hubs can pull the flanges right off. Of course some of us know how tight to build them but..., don't try this at home.
