Scientific theories EXPLAIN why certain aspects of nature act as they do. Furthermore, depending on the specific theory, the aspects of nature they describe can be subject to variability according to the conditions.
For example, Newton's LAW of gravity DESCRIBES, via a mathematical expression, gravitational action caused by a mass.
Einstein's theory of relativity EXPLAINS how gravity occurs via a warping of space/time by a mass. It also explains that time can change for an object according to how fast it is going.
Furthermore, Einstein's theory of relativity has been validated in spades by numerous tests, and it has been used in numerous technical advances. Even your GPS would not be as accurate as it is if it did not incorporate Einstein's theory of relativity in its calculations.
In fact, Einstein's theory of relativity shows that Newton's LAW of gravity is NOT accurate under certain conditions.
So, which is superior?
But Einstein's theory of relativity will ALWAYS be a theory and never become a law no matter HOW much additional evidence is found in support of it. That is because as an explanation it will REMAIN an explanation.
Evolution is an aspect of nature. The theory of evolution is an explanation of how evolution occurs. The process of evolution is highly variable and changes according to conditions and the random mutations that occur. Therefore, since scientific laws deal primarily with aspects of nature that are invariable and can therefore be expressed by a mathematical expression that always produces the same result, and evolution is variable and random it does not lend itself to being a law. Instead, the theory of evolution explains how evolution occurs as a result of, primarily (and in its short form), natural selection of beneficial mutations.
A scientific theory must be able to be tested and to make predictions about what will be found in further investigations. The theory of evolution has passed numerous such tests. For an example, see my answer to this question.