Why do we say "acid rain" and not "acidic rain"?

We say acid rain, an open compound, rather than acidic rain, a common noun modified by an adjective, because of this:

Source: Wikipedia

This used to be a conifer forest in the Erzgebirge (Ore Mountains) spanning Germany and the now Czech Republic, killed off by what scientists call acid deposition, when various air pollutants combine with rain to form sulfuric and nitric/nitrous acid solutions. The popular name in English for this phenomenon is acid rain.

Because of CO² going into solution, all rainwater is slightly acidic, so merely saying acidic rain is somewhat of a tautology, though something like “hyperacidic rain” would not be. The term acid rain, however, is now generally restricted to the pollution-caused variety deadly to plant life. This usage is not a new one:

From Mount Vesuvius there recently came a great quantity of vapor which was saturated with chlorohydric acid… The vapor, it appears, became condensed and fell in the form of acid rain, which speedily burned all the vegetation on which it fell. — Herald Democrat (Leadville CO), 3 Aug. 1902.

This would suggest parsing the open compound as acid (n.) (in the form of) rain (n.) rather than an adjective + noun.