Simply put, fat is the most concentrated form in which to store energy. There are two reasons for this. Firstly, the body can get more energy per gram of fat than it can with similar carbohydrates or proteins. The complete oxidation of triglycerides yields 37 kJ/g or more (depending on the type of triglyceride), compared with about 17 kJ/g for both carbohydrates and proteins. (More energy can be plundered from fats because the relevant carbons are in a more reduced state; oxidation therefore releases correspondingly more energy.)
Furthermore, fat isn't hydrophilic. This may sound irrelevant, but glycogen's hydrophilic nature means that it binds about 2 grams of water for each of its own grams. Fat, on the other hand, eschews water and this allows it to be stored more efficiently - when it comes to packaging, water doesn't have to tag along.
Put together, the fact that fat is more highly reduced and is also anhydrous means that fat contains about 6 times as much energy per gram than carbohydrates do. That's why it's a clear favourite of the body's when it comes to storing energy.
Reference: Biochemistry (3rd edn.), Mathews, van Holde, Ahern