First of all, there must be a mistake in the title. Absolute Zero is not -273.15 Kelvin, but it is -273.15 Celcius. Now, you may wonder, how do people get that number? What is actually the meaning of Absolute Zero? In order to understand why and how absolute zero is -273.15 Celcius, we must first understand the basic concept of heat and temperature.
What is temperature? Temperature is how people measure the speed of molecules in an object. Molecules are constantly moving and the faster the molecules move, the higher the temperature is. How do people get 0 Celcius or 32 Fahrenheit for freezing point of water and 100 C or 212 F for boiling point of water? It is simply because the creator of those scale chose that number. Celcius (the person) determines his scale by making 0 the point where water freezes and 100 the point where the water boils. Unfortunately, even in 0 Celcius, the molecules of water is still moving, although much slower compare to when 10 Celcius or 20 Celcius. Now, if we ask, what is the lowest temperature possible? Then the answer is when the molecules completely stop moving. What temperature is this? Kelvin put this temperature as 0 in his scale and scientists have observed that it is in fact -273.15 C. As temperature only depends on the speed of the molecules, it is therefore not dependent on the quantity. 1 gallon of boiling water will have the same temperature as 1 litre of boiling water because the molecules of the two of them move at the same speed.
What is heat? Heat is actually an energy, and it is measured in calories. People constantly defined “heat” wrongly and they thought of heat as temperature. Actually, the two of them are very different in nature. Heat takes into account the quantity of the object. Imagine a gallon of boiling water and a litre of boiling water. Of course the gallon has more heat energy because there are more molecules in the gallon of water compared to the litre and they there molecules move at the same speed. When people feel something, let just say for the sake of argument, water, which has a temperature of 70 C, they usually thought it is hotter than water which has temperature of 60 C. This is simply not true because you did not know yet the volume of water. Heat, like I say before, is an energy, and it depends on the quantity of the object and this is the part where people often wrongly-defined it.
In conclusion, absolute zero is -273.15 Celcius or 0 Kelvin is simply because the creator of the scale put it that way. Celcius did not realize it at that time that heat is an energy that depends on the speed of the molecules and also the quantity of the molecules. He did not realize that there exist the minimum temperature possible (which is -273.15 C) where the molecules stop moving. Therefore, he only base his scale on the freezing point of water and boiling point of water. That is how we get Celcius scale. Kelvin, however, realize it at that time and so adjust it so that his 0 in the scale is the minimum temperature possible and indicate that there is no movement presence in the molecules.