Here's why:

* Cost: Pure nickel is much more expensive than copper. Using a nickel-copper alloy helps keep the cost of production down.

* Durability: The alloy is more durable than pure nickel, making it less likely to wear down quickly.

* Magnetic properties: Pure nickel is ferromagnetic, meaning it's attracted to magnets. The alloy used in nickels is only weakly magnetic, which is helpful for coin-operated machines and vending machines.

The specific composition of the nickel alloy has changed over time, but it's typically around 25% nickel and 75% copper. This is why nickels have their characteristic silvery-white color.