A few years ago this was looked into by Channel Ten Australia at Rally Australia. On the first run obviously the first cars are at a disadvantage as they need to sweep the surface clean but most drivers do stick to the lines and then there is the problem of ruts. When driving through the second time, the stage gets churned up even more and the ruts become deeper so you need to be careful not to bounce out of them as as soon as you get out you are in the loose gravel your in trouble as there is no grip there (Ala Possum Bourne - Rally Aus 2002 i think..). Also I remember an interview with Nicky Grist and he said once there is a clean line, bigger rocks can come out from the base of the road. This exposed rocks would be easy to break suspension on or even just a puncture.

These might be some of the reasons why perhaps???