BP's primary food source in the wild is rodents, thought they may have been reported to consume some in the wild, they aren't considered to be among the lizard-eating species of snake, so they will be apt to take those over the frogs. Also, some of the risks involved in trying to feed different prey items is the possibility of the animal being stuck on them. As everyone mentioned as well, WC animals are loaded with parasites and disease, its not worth risky your animals health
Snakes aren't like omnivores or herbivores, they don't thrive on variety in their diet. They can do fine on a single prey item