Here's why:

* Alpha Centauri is a star system: It's the closest star system to our own, but it's still very far away. It's about 4.37 light-years from Earth.

* Mars is a planet in our solar system: Mars is a relatively close neighbor to Earth, orbiting the Sun.

Think of it like this: Our solar system is like a small neighborhood, with Earth and Mars being houses on the same street. Alpha Centauri is like a different city, very far away.