Radial spokes put the most amount of stress on the flange, and combined with the high tensions of the drive side spokes that is where you can run into trouble with the drive side spokes cracking flanges.

Also, a radial spoke can not transfer torque from the hub to the rim. When you put pressure on the pedals you are putting a lot of torque to the hub, so why would you ever want a radial spoke on a rear wheel? Even if one side can transfer a lot of that torque you are still missing out on the 15% or more that the non crossed side can provide. Double crossing the spokes on both sides will make a stronger and stiffer wheel. If you recall Zipp used to lace with the radial drive side, but after feedback from athletes who demanded a stiffer wheel that responded better they changed to 2X lacing on both sides.

Lacing with radial spokes on the rear wheel offers no advantage at all, with the exception of making it much quicker to tension as you don't have to worry about the under/over spokes and making tension consistent with those.