Because a characteristic of the English language is that we shorten words whenever we can do so without changing the meaning. Fifteen years ago, we would always say "a two liter bottle." Today people just say "a two-liter." If we have to talk about kilometers, we call them "klicks." We're not quite as bad as Australians, who talk about abos and barbies (aborigines and barbecues), but we're getting there.

"A calorie is a unit of measurement for energy. Calorie is French and derives from the Latin calor (heat). In most fields, it has been replaced by the joule, the SI unit of energy. However, the kilocalorie or Calorie (capital "C") remains in common use for the amount of food energy. Definitions for calorie fall into 3 classes: The small calorie or gram calorie approximates the energy needed to increase the temperature of 1 gram of water by 1 °C. This is about 4.184 joules. The large calorie or kilogram calorie approximates the energy needed to increase the temperature of 1 kg of water by 1 °C. This is about 4.184 kJ, and exactly 1000 small calories. The mega calorie or ton calorie[citation needed] approximates the energy needed to increase the temperature of 1 tonne of water by 1 °C. This is about 4.184 MJ, and exactly 1000 large calories. In scientific contexts, the name "calorie" refers strictly to the gram calorie, and this unit has the symbol cal. SI prefixes are used with this name and symbol, so that the kilogram calorie is known as the "kilocalorie" and has the symbol kcal. In non-scientific contexts the kilocalorie is often referred to as a Calorie (capital "C"), or just a calorie, and it has to be inferred from the context that the small calorie is not intended." Source and further information: http://en.wikipedia.org/wiki/Calorie