Why we aren't making Darwin's theory a Law
Scientific Laws differ from Scientific Theories in that Laws do not posit a mechanism or explanation of phenomena. Scientific Laws are merely distillations of the results of repeated observation. As such, a law is limited in applicability to circumstances resembling those already observed, and may be found false when extrapolated. 

Examples are like
Ohm's law only applies to linear networks, Newton's law of universal gravitation only applies in weak gravitational fields, the early laws of aerodynamics such as Bernoulli's principle do not apply in case of compressible flow such as occurs in transonic and supersonic flight, Hooke's law only applies to strain below the elastic limit, etc. These laws remain useful, but only under the conditions where they apply. Such laws do not explain how and why they work.

https://en.wikipedia.org/wiki/Scientific_law

Darwin's Theory of Evolution in contrast contains both observation, and a proposes an explanation of the phenomena of species formation.