When your breed is one of the more popular breeds you're more likely to see poorly-bred dogs with poor temperaments. Irresponsible breeders see a huge market and produce dogs without any care for health, structure, or temperament, knowing that people who haven't done their research won't know the difference between buying a well-bred dog from a reputable breeder vs buying a poorly-bred dog from an irresponsible breeder with questionable practices/breeding philosophy.
I agree in that most of the negative comments you see online about Cockers are aimed at American Cockers. Honestly, it is still difficult to find an American Cocker with a nice temperament. Even in the conformation ring you'll often see dogs that are fearful and ill-tempered, yet they are still rewarded with titles. There are many breeders who do produce temperamentally sound American Cockers, but there are many many more who simply don't care about producing quality dogs.