Here's why:

* Nickel is expensive: Pure nickel is a more expensive metal than the copper-nickel alloy used in US nickels.

* Alloy properties: The 75% copper, 25% nickel alloy used in US nickels provides the desired durability, resistance to corrosion, and a distinctive silvery-white color.

While the name "nickel" suggests the coin is made of nickel, it's actually a copper-nickel alloy.