Curiously, what reasons were listed?I checked it out and it was not advised to use the pour on.
Most "pour-ons" are for haired cattle.. and it is not advised to use it on cattle, with heavy winter coats due to not being absorbed. Most sheep are wooled that they advised the drench for... That is preferred over injectible so that the wormer goes DIRECTLY to the source. Granted, there are more and more "haired sheep"... but is seems that sheep, of all the mammals, with many/most being wooled , the absorption rate as @rachels.haven mentioned is different. Many wooled sheep would not get the correct amount and their skin seems to be more sensitive to it. Talked to the vet about it years ago when we had horned dorset sheep... because we also had cattle. And he adamantly said NOT to use pour on. Again, they were wooled sheep. I think that haired sheep would be different... and there are some classes of cattle that they suggest to not use some wormers for as to them having sensitivity to.Different transdermal absorption rate is a good one. They could get not enough active ingredient or you could kill them from too much because their skin is less/more vascular and permeable than the target species.
Go by label instructions whenever possible.