Originally, a few different reasons kept lights from being installed in freezers:
1. Bulb temperature - The freezing bulb's dramatic shift in temperature (freezing to burning hot) would wreak havoc on the bulb and require frequent replacement. If the bulb were to last, the residual heat created during it's illumination would counteract the point of a freezer - to freeze. This has been solved in new appliances by the use of LED lights. LED's use less power to produce light and as a result they don't get hot as easily or quickly.
2. Frost on the bulb - Self-explanatory.
3. Confined space - Freezers are designed to hold multiple items in a confined space via (typically) solid shelves. Because of this, real estate is limited by tightly packed food items. The use of a light in a freezer with solid shelves might illuminate the compartment the light is installed in, but not much else. Refrigerators utilize wire, or clear plastic shelves allowing light to travel freely.
4. Frequency of use - Most freezers are primarily used for long term food storage, making the amount of visits to it far less than the refrigerator unit below.
With developing kitchen technology - and an excuse to charge some more dolla' billz for your appliance - most refrigerator manufacturers have begun installing LED lights in the freezer compartment as well, especially in freezer-on-the-bottom setups where solid shelves are used less frequently.