Why is bernoulli's principle a principle and not a law?

There is confusion over this because "law" and "theory" mean very different things in every day life compared to science. A "law" is just an outdated term for "theory." Theories are explanations of a phenomenon that have undergone rigorous experiments by scientists. A law is no more scientifically valid than a theory in science.

A principle is usually more specific than a theory. For instance, you have the Theory of Quantum Mechanics and within that, you have Pauli's exclusion principle.