Why is Einstein's description of gravity called a theory, while Newton's is a law?

Einstein's description of gravity is known as the General Theory of Relativity, and it is called a "theory" because in scientific terms, a theory is a well-substantiated explanation of some aspect of the natural world that is acquired through the scientific method and repeatedly tested and confirmed through observation and experimentation. Theories are intended to explain not just one set of phenomena but many related phenomena, and thus have a broader scope.

On the other hand, Newton's description of gravity is encapsulated in his Law of Universal Gravitation.

In science, a "law" is a statement based on repeated experimental observations that describe some aspect of the world. It typically will describe how things happen rather than why they happen. Newton's Law of Universal Gravitation mathematically describes the gravitational force between two objects but does not explain the mechanism of the force's action.