Here I go out on a limb again.
The AKC promotes the "sport" of pure-bred dogs. The Westminster Kennel Club dog show is the oldest annual sporting event in the country except for the Kentucky Derby (I believe - I know it's a horse race).
They also inspect kennels for record keeping, conditions, etc.
The vast majority of other registries simply maintain a data base using AKC records.
Obviously this is just my opinion, but what I see happening is that the AKC has been able to weed out the majority of breeders who are operating what we would call puppy mills and the puppy mills have had to turn to the alternate registries to offer papers on their puppies.
Some people just don't care about registration at all, don't care to participate in the sporting end of it, and may not be using AKC services just because bloodlines and dog shows don't matter to them. I can't relate to them, but I respect their priorities
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CJ