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It is a scientific theory
If by "why" you mean "what," a well tested framework that explains a wide range of observations in science is called a theory.
A well tested explanation of a wide range of observations is known as a Theory, such as the Theory of Gravity or the Theory of Evolution.
A theory unites and explains a range of observations.
A theory is an idea system with a lot of evidence to support it--for example, the theory of evolution. A law (in science) is an idea system with proof attached to it--no contradictions--example, the law of gravitation.