The Short Answer: Its existence.
The Long Answer: Pianos are designed to be at a certain tension. The average piano has 230 strings, each with a pull of 165 pounds of tension, which equals a combined pull of approximately 18 total tons of tension! Like anything at a high tension, piano wire, if not maintained at its designed tension via regular tunings, will naturally release tension bit by bit, resulting in the piano going out of tune.
As such, this makes tuning the most important form of regular maintenance for a piano, as consistent tension is good for the stability of the overall physical structure, not to mention the optimal sound, of the instrument.
The Even Longer Answer: Changes in temperature and humidity, since pianos primarily consist of wood, steel, and iron, all three of which are extremely sensitive to changes in temperature and humidity.
Therefore, to keep a piano from going out of tune as much as possible in between service calls, the instrument should, if possible, be kept out of basements and garages, and away from windows, drafty areas (i.e. any door leading outside), direct sunlight, any heating source (including fireplaces, radiators, heating vents, and air conditioners), as well as rooms that frequently have fluctuating levels of temperature and humidity such as kitchens and bathrooms.
And Finally: Frequent playing, which causes the strings to vibrate and move every time a note is struck, which therefore incrementally changes its position, often necessitates more frequent tuning.