It is because dogs that eat raw meat are more likely to be carrying salmonella or other bacteria in their systems.
I don't know how AAFCO performs their feeding trials, but I've done feeding trials in school. We tested the different contents (fat, protein, minerals, ash, etc) in the food and then in the feces, to compare what went in with what came out. This would tell us what got digested and absorbed. Vets would do regular exams and bloodwork on animals to make sure they were healthy. It was a lot more than just seeing who stayed alive.
I personally don't think a raw food diet is better than a high quality dog food (key words: high quality. I'm not talking about Kibbles and Bits or crap like that). I've read a lot about the pros and cons of raw and commercial diets, while trying to decide what to feed my dog. I've also talked to vets and animal nutritionists. As in everything, there are a lot of contrasting views. I've taken many animal nutrition classes for both small and large animals (and not any that were related to any dog food company), but I am certainly no expert. I always ask for many opinions from people with different backgrounds before making a major decision about my animal (and I consider food being one of the most important decisions when it comes to pet ownership) because I want to get as many viewpoints as possible before deciding what I want for my dog. It seems to me like both raw and kibble can be equally damaging. Commercial diets are formulated to meet a dog's nutritional needs, but they contain additives and preservatives that can be dangerous. Raw meat you find in a store is not the same as the raw meat a dog would get after a fresh kill. It has been exposed (either directly or indirectly) to chemicals and bacteria while being processed. It is more likely to contain salmonella, ecoli, etc. Plus, domestic dogs are NOT wolves. They've been eating cooked foods for many of their generations, so not all dogs can handle raw meat as their digestive systems have been adapted to cooked food.
All I am trying to say is that it doesn't make me a bad pet owner because I don't wish to feed my dog raw meat for personal reasons that relate to my dog. I'm not trying to argue because everyone is entitled to feed their pets what they wish and all diets have their strengths and weaknesses. Plus, not all dogs are the same. One dog can thrive on one diet while the same diet could make another dog sick. Maybe the ideal diet is mixing kibble with some raw meat and some cooked, who knows?