Because science has specific terms for things and law is not a measure of goodness or finality. A law is a set of observed facts and theory explains why those observed facts happen.
A LAW If X happens then Y will happen. Drop an apple? It will fall. That is Newton’s Law of Universal Gravitation. The law states that the attraction between two objects is a property of their mass and their distance apart.
A THEORY explains the laws. Why does the apple always fall toward the earth? Because Einstein’s Theory of General Relativity which explains gravity as a property of space and time. Here the two objects bend the fabric spacetime and these “divots” merge toward each other.
Scientific Method
The scientific method goes something like this:
We observe that things happen. For example, every time we drop something, it falls to the floor. We start measuring things like “Do things always fall at the same rate?” We conduct tons of experiments to measure acceleration and mass and distance. From that, we can come up with a hard and fast law stating:
Then we ask ourselves – why does this happen? What causes two things to be attracted to each other and move toward each other? And then we hypothesize some reasons why this may be so. Here, Einstein had a thought experiment about what if the fabric of the universe is like a sheet, and each mass within it created divots in that sheet. What would happen as they got closer to each other? Then we observe, observe, observe, and see if our observations match the hypothesis. If so, then it becomes a theory. If not, back to the drawing board.
In the case of the Theory of Relativity, the solar eclipse in 1919 provided a method to see if starlight from distant stars would be bent as it moved close to our star. The “Eddington Experiment” showed that starlight did bend in the way predicted by the Theory of General Relativity.
Just because the theory was proven correct, it did not become a law because it explained why gravity works. Not what is happening.
For evolution, the scientific method went something like this:
Darwin observed animals that fit perfectly into their niche and that specific parts of anatomy performed specified functions. He came up with a hypothesis that things evolved via natural selection – where a mutation occurred that gave an animal or plant an advantage over other animal or plant. Because of this advantage, they survived longer to breed more and pass on more of their traits.
Overtime, and with more observations, this solidified into a theory of Natural Selection. Which has been documented again and again.