Why Isn't Evolution Considered a Law?
This is an issue which often confuses the general public, as the two words, theory and law, have very different common meanings. But in science, their meanings are very similar. A theory is an explanation which is backed by "a considerable body of evidence," while a law is a set of regularities expressed in a "mathematical statement." This is why Newton's Laws of Motion are referred to as laws and not theories. They are expressed with simple equations (like f = ma for his 2nd Law of Motion). Evolution, and most of Biology, cannot be expressed in a concise mathematical equation, so it is referred to as a theory. A scientific law is not "better" or "more accurate" than a scientific theory. A law explains what will happen under certain circumstances, while a theory explains how it happens.
[ References: Kennesaw State University: Scientific Laws and Theories ]