In most upright freezers the thermostat is mounted inside the refrigerator/freezer where you have to open the door to access it keeping the whole thermostat the same temperature as the inside of the refrigerator/freezer. But in most chest freezers the thermostat is mounted on the outside with the sensor tube running inside to the coils. This allows cold outside temps to cool the thermostat too much shutting off the compressor.
Then, since the freezer is well insulated, the inside will eventually warm up and defrost as long as the temperature on the outside stays low.