When constructed in the 1800s, aluminum was a semiprecious metal, both hard to find and difficult to extract from bauxite, also known as aluminum ore.
So at the time, it was considered a fine luxury to have an aluminum cap crowning the top of the monument.
Still, there were practical reasons as well.
For starters, aluminum was safer than using stone because it was of lighter weight, adding less pressure to the entire structure. For another, the aluminum acts as a lightning rod.
As a result, no other structure in Washington, D.C. was allowed to rival the Washington Monument in height.
Considering the number of thunderstorms that rumble through the area, giving the structure a metal tip seemed like a mighty good choice.