The smelteries use hydropower, and the power wasn't just laying around, each smeltery requires building a massive dedicated hydropower plant.
At the beginning of the 20th century we went on a bit of a dam building spree up there (and all over, but heavily up there). Then when WWII broke out and we got a huge demand for airplanes (which require light aluminum), the industry congregated around where there was an excess of cheap hydro. Hence Boeing, Lockheed, and friends all being out of Seattle.