because The temperature of a body is a measure of the energy of the submicroscopic random motions of the constituent molecules. The minimum possible energy of motion is zero: no motion. We call that absolute zero. In quantum mechanics there’s always some motion, so zero becomes the minimum possible energy of motion of the body. This minimum energy can be altered for example by an external magnetic field, so the zero of temperature is not really perfectly absolute. Depending on what you call zero, changing external conditions might make the real minimum energy a little more or less than that.
