Why is Evolution Just a Theory?

One question that came out of the recent Evolution debate with Bill Nye and Ken Ham was the concept of evolution as "just a theory."

Why do we refer to some things as a "law" and other things as just a theory? Think of it this way: A theory is something that describes a system using all known evidence but is expected to be revised as our understanding changes. An example is atomic theory. We have a vast understanding of how subatomic particles work yet every day yields more information. We have germ theory which is refined by researchers regularly as new techniques are discovered.

Similarly, we have the theory of evolution. We know that it happened but don't have a perfect understanding. The best way to understand what a theory is is as an explanation of facts. The number of facts to explain will increase over time which means the theory will also change. One important thing to remember when one says "the theory of evolution" is that evolution itself is regarded as a fact. All of the mechanisms for natural selection are not yet fully understand and likely never will be. Evolution will never be considered a law.
That begs the question "what is a law?"

Laws are the mathematics that explain a given situation. For example, we have the laws of thermodynamics. These are equations that describe thermal systems. Perhaps a better example are Newton's laws of motion. These describe most systems; however, not all. Newton's laws must be supplemented with quantum mechanics as Newtonian mechanics breaks down when objects are moving very fast or are very small. Discovering the bridge between Newtonian and quantum mechanics occupies physicists to this day!

Short version: We call it evolutionary theory because it is the theory of natural selection that explains the collection of facts of evolution.