There is a lot more to this story, but briefly, the pH of the mouth. The more acidic and the longer it is acidic, the more likely you are to get caries.

Also, just because Diet 3 is a lot better than Diet 1 (and 2 for that matter), that doesn't mean that Diet 3 is the ideal/best diet. We would need to research this more using Diet 3 as the starting point.