Nathan Heller calls its roots "unremarkable and un-American":
The pies of the ancients, rather than being oozing desserts, were combinations of savory foods baked in a pot made of tough dough. … This crust-pot baking method spread through Europe and gained popularity through the Middle Ages, since the dough shell, called a bake-meat (later, just as appetizingly, a coffin), allowed meats to stew without losing moisture. It also helped seal off the meal and slow down spoilage.
The Dish 