Dr Acland tells us it is a very old mutation, probably around before our ACDs were even ACDs, back when labradors and poodles and ACDs were not even separate breeds, perhaps over 50 generations ago and before. Think about that. So the dogs that went into making up our breed went into all the dogs that we know as ACDs. It just happens that most likely a dog, with other attributes breeders at that time liked, also had prcd or carried a gene for prcd (the specific type of PRA that breeders are trying to find specific markers for) and passed it along to some of all of its offspring. Another dog that was a carrier or affected passed its genes along. With carriers being bred, only one in 4 pups would be affected, so with a late onset problem, it could take years to figure out that a line was affected with the recessive gene. Some lines probably do have more of a problem by way of having used an affected, or several versus some lines, having used only a carrier or 2, but few people are left untouched by this problem.
Probably a common ancestor that was greatly used (or more than one) introduced prcd and now it is a problem. Continuing to widely use more affected dogs could have caused a greater penetrance of the problem in the breed as a whole. "Little Logic" (born in 1939) was probably a carrier or even affected, this being that a recent dog that was behind so many of today's stock must have spread the problem early on.
If one goes to any pedigree program and clicks on a random dog, then continues clicking back through the pedigree, most dogs will at some if not many points, go back to Little Logic. Try it and see where you go. In fact, you will begin seeing the same dogs highlighted again and again the further you click back. ACD's do not have a large gene pool!