Because it IS. Have you ever seen it? Just look up in the sky at it. Even the ancients with no telescopes could see it's red or at least pink-ish. Go out at night on a really DARK night (as ancient people had with no street lights) when it's above the horizon and it's very obviously a red point of light.
Never mind why it's red, the fact is that that's what you see.
That's even the reason for its name. Red looks like blood, you get a lot of blood in war, the ancient Roman god of war was Mars. Simple.
The equivalent in Greek mythology was Ares, so the equivalent of geography on Mars is areography.
And the word "planet" comes from the Greek "planetes", which means "wanderer". Planets are close to us on the cosmic scale, unlike stars, so they clearly move from night to night. So according to the ancient Greeks and Romans, it's one of those because it moves relative to other stars, it's clearly red, of course it's the red planet.