They were just that good at growing it. Hell, the ditch was probably chosen because it had the best conditions. It might have had the right moisture, soil, airflow, and sunlight for it. There's a reason why it's often grown indoors or in controlled environments, even when it's perfectly legal.
Old trash cans and even sturdy bags work for potatoes if you can't find big pots. Clean pallet wood that has only heat treatment can also be used to make planters that last a few years.