Aluminum manufacturing is very energy-intensive and requires large amounts of electricity, which the other comments have covered.
Washington State and the Pacific Northwest has an abundance of inexpensive electricity due to the large-scale hydro-electric power projects installed from the 1930s through the 1970s. Examples include the Grand Coulee Dam, Chief Joseph Dam, Rocky Reach Dam, and more. Today the power distribution is covered by the Bonneville Power Administration, which distributes its generation to the local utilities.
As these major electrical producers came on line with relatively abundant and cheap electricity, it spurred aluminum refineries to be built in the area, which then fed refined aluminum to the local aircraft industries during and after WWII. The Seattle area had very significant activity in aircraft production (Boeing) and ship-building (numerous shipyards) during WWII. Aluminum refineries were also located in Spokane (Kaiser).
One could argue that this helped set up a technology, semiconductor and later software base in Washington, as each industry led to the other.
A side benefit is that the dam projects also irrigate central Washington, turning this desert area into highly productive farmland. This is called the Columbia Basin Project.