Why can't you buy deer, crappie, rabbit , brim etc in stores?

Depends where you live. Right here in FL, I can buy ostrich, alligator, buffalo, goat and rabbit. You have to be careful with wild game because some of it can be infested with parasites like toxoplasmosis, etc, and is unsafe to eat. The Dept of Agriculture is responsible for inspected the food we eat. Look for an inspection sticker when you buy. If you want freshwater fish, get a fishing license and go catch some yourself! They taste better that way too! ;-)