Because animals are simple models, and humans are more complex. Also, in the 1800’s it was generally accepted that putting a human in a cage like a dog, in order to study how they react to a single stimulus would have been unethical at least, and likely torture. To make sure his test subjects weren’t reacting to previously learned stimulus, he needed animals who didn’t have much history. Young dogs, in pavlov’s case. Using young dogs is relatively easy, but using young human children has many more ethical issues. During his research on the physiology of digestion in dogs, Pavlov developed a procedure that enabled him to study the digestive processes of animals over long periods of time. He redirected the animals digestive fluids outside the body where they could be measured. Does that sound like something you and your family would like to sign up for? Being in a lab for months and surgeries to have your stomach fluids and saliva go through tubes, but consider that he would have had to use children. Would you rather your own child or a random dog?