Having the main freezer on the bottom is insanely inefficient. Hot air rises, and cold air falls. When a freezer is on the top, all of the cold air is generated in that freezer, and then it falls through a channel into the main refrigeration area. It keeps both compartments cold using just one system.

Freezer on the bottom requires either a separate cooling system for the main fridge, or a series of fans that constantly draw cold air upwards, wasting electricity in the process.