Pickerel frogs are likely not used in large numbers because they're toxic [that latter feature is mentioned in field guides 50 years or more old]. You can keep large numbers of leopard frogs in a single holding tank. Do that with pickerel frogs, or mix them with leopards, and you'll likely have a lot of DEAD frogs.
Amphibians in general are toxic. It's a matter of what ways they are toxic. Many of the most toxic contain alkaloids in the skin, which derive from alkaloids in prey invertebrates, which in turn obtain them from plants. Many other toxins are produced by the amphibian. Toads can produce massive amounts of serotonin - a neurotransmitter, but one which is lethal in you swallow large amounts of it. They, and many other species, produce a plethora of toxic chemicals. You don't have to be a poison frog eating toxic bugs to be toxic.